Familiarize yourself with every champion (at least know what CC they have, and about how much damage they're capable of), and then find something you can have fun with and try to learn it's more subtle tricks, techniques, and tactics.

It is great to play many champions. Knowing what they do is useful when playing against them, and it makes countering easy. The problem with this is that there is little time to get good as champs like oriana, or elise (high skill cap champions).

I think elo-wise it's best to attempt to perfect a small number of champions...but personally, while its fun to become decent with a large number of champions, I'd be more inclined to try and master a small number of champions that I could play with more competitively.