If you want back history and old stories that are severely out dated, go with Judas Contract and some of the Wolfman trades. If you wanna keep up with what's going on in the current Teen Titans world, Titan's Tomorrow is a good jumping on point, and just keep going from there. It's not exactly the best arc, but it does explain why a lot of the members leave, have their traits they have now, so on and so forth.
I've been a fan of pretty much the entire OYL run. While some of it is stupid and theres some glaring mischaracterizations, it's still a comic, and a pretty damn entertaining one at that.