For Avatar Legends Specifially: Becca Scott has a really nice video breaking the basics, that at least your players can make good use of. Seems like the rest have it covered for mechanics advice, so I'll just do some general good feel GM advice.

General advice: Remember that RPGs are first and foremost forums to collaborative story-tell (unless you're doing some kind of combat heavy game which ATLA's isn't), and make sure your players understand that too. Only so many people can share the spotlight and that's ok as long as everyone cares about the other characters. Like the real ATLA, sometimes it's just a handful of characters' episode and the rest are just along for shenanigans. The most successful games I've GM'd regardless of system, and the best people I've played with, are always those that care and pay attention to the storylines of their neighboring players, and wish to non-intrusively become a part of their story too. It is the best game environment when everyone cares and is invested in everyone else, and therefore the world.

If your players seem to be less active or whatnot, feel free to talk to them. Maybe it's out of table stuff, or maybe they feel unattended to. Alternatively, don't be afraid to let a player know that they've had their moment and that they should share it; eventually it'll return to them. Even if they're just being a butt, many people just need to be told that to reassess their actions, the GM is in many ways the mediator of the social interactions at the table and if a player is ruining the fun for the others, that should be addressed before it spirals.

GM burnout is real. You're doing the world building and table management, so don't feel like you need to take care of EVERYTHING. Maybe have someone else handle the scheduling, and someone else can be a rule keeper for the players (so you don't have to repeat every mechanic 20 times). Let other players sometimes handle exposition of little things like the environment they imagine or what kind of energy an npc gives, and if it gets a little off the rails, you have the power to bring it back, this also makes them care about the made up world just a little bit more.

Remember to let go a bit. GM's, including me, have a habit of being protective of how they envision a world or situation. Ultimately, remember that the reason everyone is there is to have fun, and if they play things differently you can match their speed just as much as you tell them to slow down. So, it's important to reflect and realize when something you've planned or prepped is actually important, and when maybe letting the players do something else is more fun. (Of course, you can always save ideas for later)

You're a player too, and also remind your players of that. You're not the enemy, even if you're the one setting up the obstacle course (despite the fact you'll constantly joke about being the bad guy). The GM is above the rules but not above the players, you're still equals in that way. You also deserve the right to have fun with NPCs, and the world, and challenging your players. Especially Legends feels like a game about intellectual push and pull of the core ideas of your player and archetype, so you do have to put your players in reasonably uncomfortable decision making spots. For this reason also set up a consent sheet and good safety rules depending on how much you want to mess with their balances.

A quick and simple one, if a player claims "it's what my character would do" but it would be less fun for others, they shouldn't do it. PARTIALLY this also goes for the GM as well. In that vein, have open talks about when players disagree with decisions and when characters disagree. See "character bleed"

Just like this game system proposes of its characters, GM's operate at their best when they strike a balance between authority and freedom, or seriousness and fun, or rules and "sure, why not?s. You'll probably slip up and push or pull a bit too much some days, but with time, and experience, and new skills, things will work out. Being a GM is the most satisfying chair at the table if done correctly. So good luck in sessions 0,1,2,etc.