Here are eleven tips for GMs in general. If you want tips specifically for the use of Roll20, you can find some at Running D&D5e on Roll20 and Running Fate Accelerated on Roll20.
Why should anyone listen to my advice?
I've run 100s game sessions on Roll20 and played in many too. Most of these sessions have been D&D 5E, but I've also run many sessions using Fate Accelerated, Dune:Adventures in the Imperium, Mothership, Ironsworn (Co-op), Call of Cthulhu and many, many more.
Outside of this Roll20 experience, I guess I'm older than most people reading this (over 60, and not yet "grown out" of-playing games) and while age doesn't always bring wisdom, I've been regularly playing and running all sorts of RPGs since 1981 and had a lot of success (by which I mean FUN).
When I say "regularly" that means at least two games a week over that whole time. For the last several years I've added to my face-to-face games with the Roll20 Virtual Table so am often doing 3 or 4 games a week. There's no way to say this without sounding boastful, but I'm a pretty good DM. Lots of people have been kind enough to say it over the years - not just my close friends, who have to say it, because I know where they live. I've run a lot of games for strangers at conventions and the key thing that convinces me I'm doing something right is that people usually come back for more. I've had a lot of repeat players over the years and been able to run some very long campaigns. On Roll20 I've managed to get together with a group of complete strangers and we've stuck together playing mainly D&D for a several years now; starting with some homebrew games of mine, then Lost Mines of Phandelver, Hoard of the Dragon Queen, Rise of Tiamat, Curse of Strahd, Tomb of Annihilation, Storm Kings Thunder, more homebrew adventures with the players as members of the City Watch.
“Stop rambling old man and get on to the advice...”
General Games-mastery Tips…
1. "Running Games is Easy"
At least it is easier than you think (if you haven't done it before). It's not quite as easy as being a Player, but once you've done it a few times it quickly stops being anything to worry about.
2. "You get to tell your own stories"
This bit of advice is not really true. You'll hear it from a lot of people, but it is one of the TRAPS that new DMs often fall into. The truth is (or should be) more like... you get to design the setting for the story and you get to present the situation and setting to the players, but you should try to remember that this isn't story-time, this is a Role Playing Game and the players get to decide what their characters do and can take the story in unexpected directions.
It's their story as well as yours - probably more theirs than yours if you have really proactive players who get immersed in the setting and want to pursue plot lines they find interesting. This should not be thought of as a problem. It’s actually a great compliment to the DM.
On the other hand, if you are prepared to run a specific scenario and a player wants to go completely off track, that's a lot easier to cope with in a low-prep game. If you've bought a fully-prepared module (like Storm King's Thunder) or painstakingly built your own monsters and maps in Roll20, you may just have to ask the player to head back in the vague direction of what you've prepared.
3. You ARE in charge of the session but it should be a "democracy" or a "benevolent dictatorship" if possible. You might have heard the term "social contract" used to describe Role Playing Game sessions. This is a fancy way of saying that when you play an RPG, to make the game work and to make it fun for everyone (DM and players) there are some basic social rules. The DM has invested the most time and effort into the game, so the DM gets the final say on most things.
It's OK for players to ask questions, to discuss interpretations of rules or discuss how rules apply and whether it is fair to put a huge dragon in room 1, but you should work on the basis that the DM has done that for a reason and trust them to not just be trying to kill your character.
It's NOT OK for players to bully the DM or insist on things if the DM rules that it works in a certain way. It's NOT OK for players to bully other players or try to kill their characters. It's NOT OK for players to deliberately cheat (frequently forgetting to knock off hit points when you take damage, claiming to have a higher Armor Class). If the DM suspects something isn't right, it's OK to talk about it - maybe it's just a misunderstanding.
4. It's OK for the DM to not know all the rules. You might have to look up some rules like "how grappling works" during a game or might say "for now it's going to work like this, but we can look it up later because I don't want to stop the flow of the game just now".
5. It's OK for the DM not to know exactly what all the powers of the monsters are. Players usually just need to know their own character's powers and spells. DMs have to cope with powers and spells for an ever-changing cast of creatures. It really doesn't matter if you get a few things wrong.
Often I get things "wrong" on purpose - adjusting the stats of a monster or giving them slightly different powers, so the players don't always know EXACTLY what the creature can do. I do it often enough on purpose that, when I do it by accident, it doesn't really matter.
6. The DM should not be trying to WIN. This one is often tricky for new DMs to get right. The bad guys (controlled by the DM) should appear to be trying to win, but real victory for the DM is running an exciting game so that players want to play in your games again. This may sound a bit soppy - "I'm happy if you're happy" - but it's kind of true.
It's surprisingly satisfying when you find out that players are looking forward to the next game and that they are asking questions and making plans, because the game you ran has them hooked.
This doesn't mean always letting player characters win, regardless of what they do, but there are some DM-tricks you can employ to reduce the chance of a TPK (Total Party Kill - where you wipe out the whole group of player characters)...
Keep in mind that the monsters don't usually know everything about the PCs so won't always make the best tactical choices. In a "real" fight against a group of PCs, it might be most sensible to kill the Cleric first to prevent them healing the others. DON'T.
It might be tactically sound to KILL a PC that is knocked out, to stop them being healed and re-joining the fight. DON'T.
The best tactic for groups of PC's is almost always to "focus fire" all attacking a single monster until it falls. In most cases for a DM's monsters attacking the party - DON'T. It is not much fun for a player to have their character knocked out in the first round of combat and might be several rounds (or even the whole fight) before they can get back into action.
Players often forget that running away is a valid tactic - the DM can remind them with a subtle hint or by saying "Don't forget you could run away". Not often though, as most players want to be heroic and cool if possible.
Players often forget that some creatures might accept surrender or a truce - the DM can remind them of that or have the monsters suggest bringing the fight to an end. Again, don’t let this happen too often or the players will get de-motivated.
If you do accidentally overwhelm the PCs, try to have a contingency plan. Perhaps a new DM just underestimates how dangerous some monster is - perhaps a dragon breathes on the party and everybody fails to save.
In the first place, if the party have at least made some attempt not to all stand huddled together, try not hitting them all at once, but if it does happen, maybe the monster wants to capture the heroes and send them on a quest? You shouldn't over-use this "You wake up in shackles" ploy, as players are almost as unhappy to be captured as they are to be killed - but it is definitely worth considering if the alternative is to start a whole new campaign.
Finally – if you do accidentally WIN – apologise and try not to do it too often.
7. Don't forget that the Players only perceive the environment through what you tell them.
I always remember playing a druid who could change into animal forms. The DM told me there was a bridge over a river and I said my druid would dive into the river changing into a crocodile. The DM reached for many, many D6 because the river was 300 feet below (and crocs are not that aerodynamic!).
That's an extreme example, but if a player risks their character's life to block the only way into a room, make sure you've told them about the three other entrances their character could clearly see.
8. Don't be afraid to say "Oops, I got something wrong earlier".
As long as it isn't too awkward, you can always roll back time and try again - or just make up something that compensates for the earlier mistake.
I usually only roll back and re-do something if it is a serious enough mistake to really matter. If you just forgot that your monster gets a special bonus damage - just start adding the damage from now on or bring in a second monster (if it's important that the current encounter is more challenging).
Remember that the players don’t know what was “supposed” to happen, so it’s probably only you that knows you got something wrong.
9. Don't be afraid to make stuff up!
If they turn left despite their map suggesting the treasure is to the right, perhaps it comes to a difficult climb or a hard-to-cross river of lava? If they push on, maybe a swarm of giant bees attack, with an extra few bees arriving every round.
If you're playing in Roll20 and you haven't got a map for the left turn and you haven't prepared the monsters for the left turn, it is OK to ask the players not to go left - "Sorry guys - I didn't prepare for that, you can sit about for half an hour while I prep, or leave the game there until next week, or decide to go right instead"
10. Prepare at least one "emergency encounter".
Eventually your players will go somewhere you didn't expect or find a clever way to bypass something you prepared and so get further in one session than you expected. It's always worth having some extra encounter prepared - a map, some monsters and some sort of reason for the players to get involved.
11. The DM is responsible for giving the party treasure.
Really this means rewards of any kind.
Experience Points so they can go up a level?
Money so they can buy equipment (but you need to have something to buy).
Other benefits in-game - a knighthood, a horse, a friendly cleric who owes you a favour.
Don't be bullied into giving too much (D&D5E is not like D&D4E and magic items aren’t REQUIRED to keep the play balanced) but try to remember to give them something. Magic items give Players more options in the game and so are cool rewards.
12. There is no tip 12, I said there would be 11, why does everyone always want more!
ENJOY WHAT YOU DO AND KEEP CHECKING THAT THE OTHERS ARE ENJOYING IT TOO!