Good on you man for trying to be a good father figure for her. I know I'm totally not ready to be a father, and it amazes me that people my age are able to do that, it definitely takes a special person our age to be ready for that. Some circumstances have made my girlfriend and I seriously look at ourselves and think about whether we could be parents in the near future, and for me, the answer is definitely no.
As people have said, make sure you and your girlfriend are on the same page about you want for her daughter, what types of morals and values you want to make sure she understands, etc. Make sure to support her and help her do the things she wants to do, if you're able. As Laura said, try to see what it is she needs the most and help provide her with that. Providing any sort of stability is great. Do what makes her happy, what makes her smile, and be a good role model.