How do you ask a girl to be your girlfriend?

Okay, there is a difference between going out and being Boyfriend and Girlfriend. When you go out you are just beginning to get to know each other; kind of like the first few dates. When you actually ask someone to be your girlfriend than you are a serious relationship and you are basically going steady. I think it is a good time to ask the girl you are going out with to be your girlfriend after the third date when you really know each other. It doesn't have to be romantic. It could be out at the front of your school or in your car. I think you should be standing though. She'll probably really enjoy it if you give her your watch or a necklace you wear; something really important to you so she can know how much she means to you. If you want to do it at a romantic spot, I would suggest a moonlit walk or at a carnival or fair (okay I know it doesn't sound that romantic but your girl will love you winning a prize for her)! Be sure to act confident and be very relaxed. Don't act jumpy or she'll be slightly scared. Hope this helps!

well first you just need to ask her.and it should be in a nice and sweet way.

What kind of relationship do you two share now? Like, are you guys good friends? Have you taken her out on dates?
Before you ask a girl to be your gf, you have to spend some time with her, and show her through your ACTIONS, that you want to be her boyfriend.
If you really have to ask, for clarification. Then make sure you are both in a serious state, and then just ask "will you be my girlfriend?"

make it casual in person like will you be girlfriend.