Her body language will tell you if she wants you to kiss her or not. The first step is to get her relaxed and enjoying herself. Then you can put your arm around her shoulder. She'll either relax into it or tense up. If she relaxes, just let her relax more. Then, turn toward her and tilt your head toward hers. She'll either melt into you or act like she's very interested in the movie. If she doesn't turn into you, she doesn't want to be kissed. At first, just gentle lip contact with some movement. Your body will know what to do. Since you're new, don't try anything advanced, like putting your tongue in her mouth. Just relax and let your body know what it knows how to do. You'll find your lips moving naturally and I'll bet the two of you will figure it out.
FYI, the kiss scene in Titanic is great.