All right, first of all, do you like playing in the standard metagame or the underused metagame? You'll also have to do some research on what Pokemon are banned in certain tiers and whatnot. I'm assuming you play the standard metagame with the big boys like Salamence and Tyranitar.
Secondly, you'll need to have a team of 6 Pokemon that can cover their weaknesses very well while being able to respond efficiently to enemy threats. List all the Pokemon you love and like to use and I think I'll be able to help you with that.
You'll basically want a lead Pokemon, physical sweeper, special sweeper, physical wall, special wall, status platform, revenge killer, wish supporter, stealth rocker, rapid spinner, spin blocker, etc...
As for me, my team relies on indirect damage and stalling to destroy its enemies (and this team hasn't failed yet), but I'm not playing with my OU team in a LONG time. If you want, we can practice in Shoddy and I can help you make a team there. I've helped two members Phreen and UnderXRay make their teams... so specify what you want