Since I'm a guy I might not be super qualified to give this advice, but I do understand liking girls.

Does she know you're not straight? If not, that might be a thing to start with. If she does, just... choose a good time to tell her you like her. It's really really scary, but I'm sure if she's the kind of person you care about, she'll react well to you telling her. Maybe she even likes you back!

I'd say just plan what you want to say to her and get up your courage. It's not easy, but there's not much of a down side if she doesn't like you back beyond a few minutes of awkwardness, and there's a huge up side if she does.

I hope this helps at least a little. 

Like Zeus said, plan out what you're going to say to her and prepare her for it beforehand. You probably want to give her time to react and think about it as well. Sorry, I'm really bad at giving advice. 

Okay, I'm probably the worst person to answer this, since feelings are kind of a mess for me right now, but here we go anyway.

First off, is this person a close friend of yours, or just an aqquaintance? Like Zeus said, if she's not a close friend, really the only downside if she doesn't feel the same way (other than her not liking you back, of course) is some awkwardness. Have something rehearsed beforehand, or get help from a friend. By that I don't mean get someone to tell her for you, I just mean tell someone else you're close to first, and ask them to help and support you through it. Good luck, I wish you the best! If you'd like to, please let us know how everything goes! *hugs*

Oh, and for the record, if you want an encouraging example of things going right:

I had a huge crush on this girl a year older than me, and I thought she'd never like me back ever, and then I asked her out and she said yes and we've been dating for like four months now! So, good things happen sometimes! There's potential for really good things happening here! 

I wish I had more advice than what's already been given, but alas, I do not. Best of luck and feel free to tell us how it went!

~Starseeker 

Everyone else has given very good advice. All I can think to add is just tell her, in simple terms. This may come before or after anything else you want to say, any questions you have... Then it’s up to her, I guess. 

Hope this helps!