Your question is a little vague...are you just casually dating a girl and now you want to cross that line and consider her your girlfriend? If that's the case, then what you should do is take her out to a nice place for dinner one night, and afterwards go someplace really nice and peaceful, such as the beach. The beach at night can be very relaxing and pretty romantic...in fact that's where I usually go when I meet a girl early on. When you get there, tell her how you feel about her, and find some way to tell her that you want to 'go steady'. Not in such simple and rather silly terms, but in such a way that she knows you want her to be your girlfiend. Don't be afraid to get intimate...i'm not talking sex of course, but just being generally intimate with her will help. If you're talking about a girl that you really like, and you think you have a chance of dating her, then just be strong and ask her out on a date. If she's a friend already then just ask her if she wants to hang out with you and go to lunch or dinner or something along those lines. It can be difficult to just go up to someone and ask them out, but its the original and still the best way to do it. Be very nice to her, and treat her well. If you go out to lunch or dinner, offer to pay. Then go for a walk together or just go someplace where you can talk and be alone together. If you sense chemistry, then make your move.

The answer MP gave is very good. From a woman's perspective always be a gentleman and make her feel special even if she's not your girlfriend yet. Don't wait till the last minute to ask her out and take her to a nice place. A nice place doesn't have to be expensive, just a place where you can both be comfortable and get to get to know each other more. If there's the chemistry, you'll know soon enough. If there isn't that kind of chemistry, then you lost nothin and she'll continue being your friend. Good luck.