How to ask her to be your girlfriend
Verdict first
Do it in person, once you already see each other regularly and it feels natural. Keep it simple and honest — the question should confirm what's happening, not surprise her.
When it's time
You've been seeing each other for a few weeks or more, you talk most days, and you don't want her dating anyone else. If you're asking to find out whether she's interested at all, it's too early.
In person, not over text
This conversation is worth having face to face. Text is fine for a playful hint — not for the actual question.
How to say it
Simple and direct. No speeches, no ultimatums:
I really like what we have. I'm not seeing anyone else, and I don't want to. Want to make it official?
So, I'd like to call you my girlfriend. How do you feel about that?
If she's not ready
Ask what she needs and actually listen. "Not yet" isn't "no" — but if months go by and it's still "not yet", you're allowed to decide what you want.
Verdict