Ou is the most common tier right now. Although tbh, it's probably the most boring.
We can help you by rating your team and giving tips on RMT, but the starting team itself it down to you. Be creative but sensible.
There are lots of ways to study and improve on a tier before even paying it. Ask people's opinions, go to smogon and see what they say personally, a lot of it is bad stuff, but some of the sets and strategies are good ones you can learn from, go to showdown and watch battles, see what is common in OU and what threats you need to be aware of, on the same note, do the same things with replays (both we and smogon have replay threads as well as Showdown's, so you can go see them too).
But the main way is to play it! Make a team (don't get too creative until you know the basics) and go out there, either against friends, friendly DB users or out on ladders. By doing this, you'll get a great sense of what is threatening to your team, what your team lacks and the playstyles that do well, as well as the ones that don't. Contrary to what people say, becoming extremely good isn't easy, and will take you some time to become elite. But don't get disheartned by losses, you can look at them and see what you could have done.
Just practice, practice and practice. I wouldn't personally say to play at all unless you're having fun, but many people have fun after they reach very high up standards, s whatever floats your boat.