I want my superhero girl to be a teleporter, but when I think about it, I can't think of many weaknesses... a little help? :(

Well, even if she is a teleporter, she'd still have all the weaknesses that normal humans do right? Unless you're going for something else a little more obscure and wacky, I think it would be safe to say that she's not impervious to things like guns, knifes, stronger people than herself, cars etc.

Things like hand to hand combat could be something she is bad at.

Without knowing more about your character I'm not sure what else I could suggest?

i think that she might forget things fast and be confused,that she teleports to the wrong moment/place etc.
i don't know much about your story either,but i agree a bit with seripha,that she isn't the best in violence,but that she's sensitive
not that she can't find!
and maybe she can have problems that noone knows her "secret"(if it is one,of course) and that she's uneasy with the boy she loves,friends and doesn't know what she can tell to lie about where she was at that time,while she was teleporting and that she had to meet with friends or something like that

In all honesty, look at X-Mens Nightcrawler. He is very much not a social person, but that's due to his weird looks. It's him in action that might help, because that's when his looks don't matter. If your girl gets into a fight, she isn't going to go toe-to-toe with many people. In social situations, maybe she is nervous cause she is different. Maybe think of the movie version of Rogue. She knows she is different, and so is very uncomfortable around others, even though her difference isn't visible. I dunno, just a few thoughts.

How about having to know where she's teleporting before doing so, lest she end up in the middle of a wall or object? Why not give her a limit to how far she can teleport in one jump (for example, the furthest away she can teleport at once is a mile. When she gets to that mile, if she wants to go further she has to re-appear and teleport again.) Why not give her power a physical side effect, like making her nauseous or exhausting her if she does it a lot within a short time frame, or if she teleports a great distance?

Well, don't have the teleporting be instant, for one. If it was instant, then when she's in a fight which someone else suggested she should be bad at she can't just zap around them until they fall over dizzy. Look for limits to put on the ability, not the character.

Does your character really need a weakness?
I understand that things can get a bit stale if you have no one for them to fight, but it's a rarely explored option to have an all powerful hero.