Question originally asked on Quora titled: What do I tell a girl when I want her to be my girlfriend?
Let me take your question "What do I tell a girl when I want her to be my girlfriend" in literal context. You are asking for ways to ask the girl to be your girlfriend when you want her to be. The key word is "what". Before I go into that, let me help you do a check on the status. Think of me with a mirror helping you look behind youself.
I would hope to know that you're already at the stage which both of you are ready. By that, I mean you've been on dates, or at least, some flirtations have been going on, and both giving and receiving has been optimistic.
One important thing I want you to know as a man is this: to be a great partner or lover, one has to know how to respect women. That's the utmost important thing, and the very minimum a man has to know and do.
An effective way to find out if she's into you will be touch. No, I'm not referring to being touchy-feely. Not that kind of touch where you'll stroke your palms down her back with your eyes staring at her with a creepy smile.
I mean to say touch her wrists. Tell her you like the watch (or bracelet) she's wearing. Hold, but not grab, her hands while you ask her questions about that timepiece. Hold on to her hands gently even after you have finished speaking. Check for her pull backs. If there's none, you have a chance.
Pay attention to her shoes. That's one of the areas men neglect. Show her you are attentive to details about her. Pick up her fork if she drops it in the restaurant, and ask for another from the wait staffs before she raises her hand.
Show dominance, decide for her in someways, but not in aggressive manners like dictating what she should do. Respect her. Show her that you're confident, and assure her that she'll be well taken care of if she's yours. Stand straight with your chest out. Speak strong, but not loud. Show her you're dependable.
The fact that you are asking how to ask a woman to be your girlfriend somewhat shows that you need a little boost in the confidence department. No worries about it though - everyone can learn to be in time.
But the golden question is: HOW DO YOU BRING THE TOPIC TO HER?
If you are certain that you've cleared what you need to do, if she's not pulling herself away from your touches, and she's laughing at your jokes no matter how lame you think they are, trust me, she's at least 65% into you. Then the question you should ask her, in total discretion (no, no. I'm not referring to hotels or anywhere sexual. That's the last thing you'll want her to feel. Parks would be good), would be - not conclusively a "question for all", but good to grab some ideas from - this:
"I like your company, and I love sharing everything with you. I'm comfortable with you, and I hope that it's the same on your side. I think, to be fair to me and not you, because it'll be in time to come, I think I would love to love you. I wonder if you'll want to give me this opportunity to be exclusively yours, and you, exclusively mine."
Prior that question above, you may want to consider bringing her to ride some roller-coaster, if she's a thrill seeker. BUT NEVER CONSIDER THIS IF SHE HAS A WEAK HEART!
Bringing her to ride a roller coaster will release a hormone called Phenyl ethyl-amine. The same hormone when one feels love. By riding a roller coaster, she'll release the said hormone that mimics love, and it'll increase your chances. The ambience when the question is asked, such as the darkness and romance often felt in parks, increases feeling of love. That'll help.
If you are not a user of pheromone perfumes, then you could use your natural scent. Wear clothes that are comfortable and permeable. Unless you have BO (you'll need a deodorant if you do), I would suggest you pop that question after a day out with her - your perspiration accumulated would increase the presence of your pheromones, and it's a subtle attraction.
The negative side is if you are more of a beta male and less of an alpha, she could sense it, and you're left with one "weapon" lesser.