You have a decent starting army as is (the starter set is actually a very good starting army already). It allows you to build a few interesting lists from 350 to 500AP. I'd suggest you keep it like that, try to play a few game to get an idea of the style you like most, and then decide on what to add. A few people usually show up on Tuesday nights.
If you really want to get new models immediately, you may want to purchase the very useful Fell Caller, or perhaps to look for another warlock (the 5 of them are very different in playstyle). For general info on models, you can check out the trollblood section of battlecollege
I play trollbloods too, I'm not a very experienced player and every game I played ended up very different from all the other ones. However I own quite a few models, so once you've had a chance to try your current models I'd be more than happy to let you borrow mine so you can taste other troll flavors.