She likes me, how do I seal the deal and not ruin it?

I know exactly how you feel. First off, confidence is important. smile a lot, and be funny, but don't try too hard. Give her flirty little comments to "test the waters" a bit, but dont be too strong about it. Just try to be natural in your interactions with her, don't try to be somebody that you aren't. Do little things that girls like...walk her to her classes, maybe open a door for her or carry something for her. As cliche as that might sounds, girls like that type of stuff. It shows your interested. Good luck!