How do I further my relationship with my girlfriend?

I'm 15 and I've been going out with this girl who I really like for almost 9 months. So far I haven't done anything with her but poke/tickle her stomach. We've never even kissed or held hands. It's actually quite sad and it's completely my fault. She says she really likes me and constantly leans on my shoulder among other things which hint that she wants me to respond to her. It's not like we haven't had enough time alone or anything...actually, one day we spent the whole day alone together. I just don't know how to go about furthering our relationship. If you've got anything helpful to say I'd be happy to consider it.