OU is the most commonly played format. Just go on http://play.pokemonshowdown.com and look at the pokemon in each tier. if you play VGC then you should already be familiar with the basics of how to build a good team and also the type matchups. If you need any guidelines for how to set up your pokemon then Smogon is a great place to go. Here is the link to their competitive pokedex which is complete with abilities, items, all moves, all type matchups, and suggested EV spreads:
PS: Expect Sun and Moon to warp the meta after its arrival.