So there’s this girl

The best way is to hang out with her(so she gets to know you, and know that you're there). Ask if she wants to come over and watch a movie, or just talk to her when you get he chance. That will show that you have interest in her. When you get a sign that she also likes you, then you should talk to her about where it could go--or simply admit that you like her! :) I think that's a pretty effective way--using proximity to show the person who you are and then see if they like you for you! I hope that helps!

Try talking to her! Talking turns to texting, texting turns to flirting, and then before long you're already dating her! Just try and have a friendly conversation first and see if any sparks happen.

Well, one issue is the question whether or not this girl even know that you're interested in girls yourself. If she isn't, she might not have even seen you as a possible partner to begin with.

Either way, simply getting to know her is a good way to start. Talk, show your interest a bit, pay attention to her behaviour and see if interest is returned. Body language is useful like that. If she doesn't return any romantic interest, try cultivating a regular friendship instead. It never hurts to have a friendly face with similar interests.