Hmmm . . . well the first 2 pieces of advice I'd give you:
1-- You're probably gonna hate this, but try playing a team of just the top 25 OU pokemon. It'll give you a perspective on the scope and power of the best in the metagame. Of course it's not a great team just by slapping OU pokemon togther, but it's just a reference point.
2-- Eventually, you want to build teams with an objective in mind. You can see I have some gimmicky pokes on my team-- Jolly Choice Scarf Skarmory has actually been really great all day (I just started using it), but the idea is to have a plan, and use pokemon that are best to execute the plan. Mine is a) Set spikes/rocks b) weaken the enemy team, and take out any ghosts (with ttar) c) Lucario Sweep. Outside Lucario, the rest of the team comes in pretty much just to support the plan.
Have a plan, have an idea of a time line (what will happen in early, mid, late game) build towards the plan, and execute. That will make team building much easier.