As a girl, I think it's kind of funny how guys pretty much always give other guys the general advice of "Make it seem like you're not THAT interested, it'll scare her."

With most girls, this is just not true.
Don't be afraid to show her you're interested. Most people love the feeling of being wanted. A basic thing that doesn't require THAT much confidence is just glancing at her from time to time. And for the love of god, glance at her face, not her body. Glance long enough for her to notice you're doing it. If you want to seem very confident, smile at her when she looks at you, or avert your eyes and pretend you weren't looking if you're more shy.
Chances are she will smile back. And even if she doesn't, she'll know you're interested. This is a neat little trick for gaining a girl's attention without saying anything. Other ways is helping her if she asks someone a question and that person can't answer, or picking up something she accidentally dropped for her.

After a while, if you're lucky, SHE might talk to YOU.
And if she doesn't, just strike up a conversation with something simple. If she's in your school or workplace, ask about something that she'll be comfortable answering. Like the latest math homework or how to make copies with the photocopier.
And don't be afraid to seem a little shy or nervous, it's okay. And I can honestly say that alot of girls find it to be rather charming. If she seems to want to end the conversation, play along and walk away. She'll find it is easy and feels safe to talk to you when you pick up on the hints.

In time, you'll gain confidence. People will tell you time after time that it's okay and it's not that bad, but since I have been - and still am to some extent- a shy person, I know this doesn't help at all. You just have to learn from experience. No one gets confident over a day, don't worry.

But this is just some advice to get some personal contact with the girl you like, not to get confident. But as I said, in time, it'll work out fine. You often get even more nervous and shy because you're worried you'll SEEM nervous and shy. Just take a few deep breaths and relax your shoulders. She's human, just like you (She IS human...right?). You want to talk to her. Would you be freaked out if a guy like you walked up and talked to you? If not, why would SHE be?

Oh, btw, that about girls liking confidence. Sure, confidence is nice, but not when guys are SO confident they seem just fine with acting like arrogant assholes. Having a guy walk up to you thinking he's freaking Don Juan is just fucking tiresome
Confidence is not at all any kind of guarantee that a girl will like you.

Best wishes, E