how to ask a girl out

How to Talk to Strangers
How to Come Up with Good Conversation Topics
Flirt a little! Relax, crack a joke, and break the touch barrier (see How to Touch a Girl).
Gauge her interest. Take notice of eye contact, smiling, laughter, and enthusiasm in her responses. Read her body language.

Notice if she touches you more often than what friends do (she’s constantly trying to touch your hand or something). If she finds excuses to do so, then you’re probably on the right track. But conversely, don’t assume that just because she isn’t touching you that she doesn’t like you. She may be too nervous to touch you yet.
Observe how she looks at you. If she likes you, she will either hold it for a long time or pull away immediately. Either of these could mean that she likes you. If she pulls away quickly, it means she is nervous but she still likes you. If you happen to glance at the girl and you see her staring back at you, then this means that she likes you, although she may quickly dart her head in a different direction.
Pop the question. "Hey, why don’t you come to the movies with me this weekend?" (It doesn’t have to be the movies – it can be anything you’re interested in, and that you think she’ll enjoy too.)

Another good way to ask is "I heard about this movie, _______. What do you think about it?" If she says she’d like to see it, ask if she’d want to see it with you. If she says "As in a date?", don’t be afraid to say yes. If she doesn’t say that, just make a time, turn up and treat it like a date and she’ll get the drift.
Keep your cool if she says no. Respond gracefully, like "No problem! Maybe another time. I’ll see you around, OK?" And go about your business – there are other fish in the sea!

Another way to go about doing it is to bail before she has a chance to respond. See How to Sweep a Girl off Her Feet.

Tips
Ask her when you two are ALONE. Having others around you will stir pressure on her to say yes or no.
Ask her out directly. Instead of texting, getting other boys to ask her out for you, or beating around the bush, just ask her. Even if you’re nervous, this will increase your chances. Most girls will admire your confidence if you are not arrogant about it.
Being relaxed and confident is the difference between a comfortable silence and an unbearable silence. It’s natural to have breaks in the conversation. Don’t sweat it.
Don’t be afraid of rejection! Most girls have big hearts and will not let you down very easily, if at all. Some girls agree to one date just because you asked and they’re being nice. Don’t take it the wrong way, it simply means she likes you enough to not hurt your feelings but not enough that she’s ready for a relationship.
When you do approach the girl, try not to start the conversation by a "Hey, can I talk to you?" or a "Can I ask you something?". Don’t ask her out randomly, but these lines are giant hints that you are about to ask her out. This would be awkward.
If she says no, just ask "Oh, OK. Is it fine if we’re still friends?"

Warnings
Be persistent, but not too persistent. If she turns you down gently, then she’s politely telling you she’s not interested. If she flat out refuses, back away. You don’t want any girl to think you’re a stalker.
If you have to ask her out through a friend, you will get a "no" by default. Girls do not enjoy hearing, "Hey! I’m asking you out for (Name of guy). He doesn’t like you enough to overcome his lack of confidence." They will hear this regardless of how tactful your friend is, unless her friend is a very close friend to both you and herself.
Make sure you don’t have bad breath!
The worst thing you can do before asking a girl out is not even having a friendship with her!

From :http://www.wikihow.com/Ask-a-Girl-Out