Hey, so I don't know how much or how little you know about VGC so I'll just give you the gist of it. VGC is a doubles format where you make a team of 6 and choose 4 of those Pokemon at team preview to bring in to the game. The Pokemon which are elligible change every year (this year there are actually multiple different rulesets for different times of the year). This year's format allows you to bring 2 restricted Pokemon in your team (things like Groudon, Kyogre, Xerneas etc., basically the cover legendaries). For the Sun series (which is the ruleset that is going on at the moment) you are not allowed to use Mega pokemon or Z-moves.
If I'm being honest, the best place for you to start is actually getting in some games and seeing how the game is played. I would recommend going on Pokemon Showdown and loading a sample team.
I would personally recommend using a sample team from here:
https://victoryroad.es/en/sample-teams-vgc-2019-sun-series/