Hey OP, I used to be similar to you...except I was shy in addition to lacking self-confidence.

Wardrobe change does help. It's superficial as all hell, but for girls, (I'm girl, too) looking your best will help you feel good - as long as you're looking good for yourself, nobody else.

If you're already outgoing and friendly, then I'm sure you have lots of friends/lots of people like you. That should be a confidence booster in and of itself.

But the one thing that helped me most?

Fake it until you make it.

Seriously. Positive thinking and believing in yourself (even when you don't really believe in yourself) really does wonders. Before you know it, you WILL be confident.

If everybody tells you you're pretty, and people have expressed interest in you (even if it's not guys), they may be on to something.

Unless you're fat. In which case, lose weight - this will also boost your self-confidence.