Man, the age of chicks you're talking about is the easiest demographic, aside from perhaps the 30-35yr olds who are past worrying about marriage. Girls who have just graduated high school are looking to explore the new world and are very open to new experiences. They can be really flaky though, so its good to meet them for a date soon after meeting them the first time, or even better, just SNL them right away!!
As for how to meet them, I find international parties and Roppongi bars to be low quality generally. You will occasionally find some nice ones, but not with any kind of regularity. The younger girls actually tend to go to the dirtier/cheaper places like Gas Panic/Jumanji or clubs with free entrance that are a little classy like V2.
But the best places to meet them are actually matsuris (not really the season anymore), school festivals (its the season now!), and the street (year round). There's nothing to worry about and nothing in particular you need to do when you talk to the girl on the street. Just remember that often they're more scared/nervous than you are, so just relax and enjoy yourself, and don't make it too wild. I think there's a lot of comments on here to be entertaining, and while that can be good you don't want to slip into "entertainer gaijin monkey" frame because trust me, that gets old fast and youll be left at the end having entertained her but not sexually interested her.
Go out, write up some experiences, and keep experimenting.