1- If possible, dont DM unless you have to. Learn the rules and mechanics. If you are interested, work with the DM to design a basic encounter and run it. It can be a very basic dungeon crawl. For example: >"our cemetary keeps being disturbed, find the culprit">investigate>follow the trail of fresh dirt or whatever>goes to a cave guarded by a zombie or something>low-level necromancer is experimenting
That type of quest is easily self-contained and wouldnt interrupt the main DMs story or DM NPCs.
2- if you are a player, make sure you know what you CAN do. There are many players who waste their characters potential by not doing their homework. Learn your abilities well and how they can interact, as well as combos with the greater party.
3- "to the squeaky wheel goes the grease". Basically, you have to make your character present. Dont sit in silence for 4 hours only to ask why no-one asks your opinion, player dialogue is integral to advancing the story and characterization.
4- DMs want a backstory they can work with, not a Tolkein-tier tome. A backstory can be functional and still be a single double-spaced page with 12-point font. Just make sure key parts are exploitable. Dont copy-paste from Anime for the love of all that is holy, because I know you browse /tg/ Trevor and I'm not accepting a character synopsis copy-pasted from the Bleach wiki.
5-Melee builds in any RPG tend to be easy to make with straightforward abilities.. but hard to put together right, especially if a caster-type class outshines you and you really need to power-game to keep up.
6- make sure you understand how initiative, damage, and defense work. Those are the things that tend to lead to arguements. Does the Defender win a tie? KNOW THAT ANSWER.
7- Know what the DM expects in terms of tone, ideally negotiated with you guys. Dont be "that guy" aka "lmao so RANDOM xD" if the table expects a serious tone. If it's light hearted romps a la a Saturday morning cartoon, relax.