I'm a STRAIGHT girl, and I have a friend who sometimes tries to flirt me. So I think she's maybe lesbian... What should I do ? I mean I don't want to lose a good friend like her...

Just tell her that's she's a good friend but that you are a straight girl and like guys. Just kind of bring it up during a conversation so she doesn't get hurt feelings. Or just say that guy over there is really cute and how much you like guys. That should work, don't you think?

If you're good friends with you, when you guys are alone and you have some privacy, you might just turn to her and ask: "Megan, I'm getting some vibes from you that seem like you're flirting with me. I don't want to put you on the spot or embarrass you because I feel we're friends. But I just want to be clear with you that while I like you as a good friend, I am interested in dating just boys, and not girls."

See where this conversation takes you. Your conversation should lead to clarify things, not embarrass her or make her feel stupid.  Good luck.

Does she seem serious about it? I jokingly flirt with my friends (any gender) and I am completely straight c: People tend to be more comfortable with their friends and can say stuff like that without it being weird.

I get that you're not most people and are entitled to your opinion, I respect that. If it bothers you, talk to her about it ^^ that or you can ignore it. Stress the fact that you like guys to her ;P

Has she shown any other signs of liking you, besides the flirting? (if I may ask)

If she makes you feel uncomfortable then you must ask her outright.  Ask her if she's flirting with you, make it a lighthearted question then see what her response is.  If she says yes she is then make it clear that you're not comfortable with that and you can't be friends unless it's clear that your relationship is platonic.

If she wants to sleep with you she's not your friend. Cut her off and see if she still wants to be your friend.

I don't care how much "thanks" this answer gets. It's the right one. Trust me.