How to take the next step?

I've always seem to have had this problem.... I feel like I've always been a decent talker/flirter with girls, able to get a few numbers and text for a while and hang out, but I can never move the relationship any farther than that. The relationship seems to stay in the limbo of talking, I don't know how to move it to the next level.... Right now I've been talking and texting to this one girl, we will both be freshman in college and this past summer we hung out a bit and went on a date ( I guess sort of a spur of the moment thing but it was only us)...we have very similar personalities and sometimes I feel like she is giving me signs of mutual attraction, mostly when we are together in person, but when we text sometimes she is really personable and sometimes she is very short. I know I like her, but I don't think she knows that. I would really just like to know how to move our relationship and other relationships to a higher level