You may have read some guides explaining EV(effort values) and IV(individual values). So now you've entered the world of competitive battling. You've bred for the best IVs and nature you can get, you've EV trained all of your pokemon, yet, you're team is still failing. Either you're very unlucky or your team's build wasn't well thought out. If it's the first reason, find a rabbit's foot, otherwise, continue reading.

The most basic team set up is composed of 2 physical sweepers, 2 special sweepers, 1 physical wall, and one special wall.
Physical sweepers are typically fast pokemon and should always have a high attack stat or some sort of attack raising item/move. You may have noticed I said "typically". In some cases the pokemon has very low speed and relies on a priority attack. Two good examples are Scizor, who has Bullet Punch, and Mamoswine, who has Ice Shard. Sweepers are used to finish off your opponent's pokemon or in other words, sweep them up.

Special sweepers are completely identical to physical sweepers except for the fact that they should have high sp.atk.

Walls are your defensive pokemon. They're used to take hits and to set up traps like spikes and stealth rock. Obviously they should have good defensive stats and shouldn't have many weaknesses. Also, they should NEVER share a weakness with any of your other pokemon. Walls often have moves like Toxic or Will-o-wisp so they aren't almost useless once they're done setting up.
Usually a wall will be physically or specially inclined, but there are pokemon, such as Milotic, who can be used as double walls.

Well, that's a basic team set up, also the most common. If that's good enough for you, then go out there and kick some...oranges? I repeat SOME. If you want to be the very best read my next blog, because I'm almost out of room on this one.