The biggest thing that holds guys back from asking a girl for their number is the fact that they COULD be "rejected" when they ask. I mean, she could have a big tough boyfriend, other people could be watching and you'd look stupid, etc. But you'll never get what you want unless you go for it.
All you have to do is talk to a girl (about whatever) for a few minutes. As long as you don't play the questions game (where do you go to school? where are you from? what sports do you play? what's your favorite food?) or look like a complete tool you'll probably get it. Just relax, talk to her about something you have in common, something in the environment, whatever comes up. Most women are really nice and will talk to you about whatever you approach them with. Sometimes you won't get the number, but it's usually because of a reason outside your control (has a b/f, in a bad mood, etc.)
After you've talked to her for a few minutes, just say "Hey it was really great talking to you, I have to get going, but you should give me your number and we'll talk." I usually give her my number so she knows who's calling when I do. But yeah, it's pretty easy. Just remember that rejection is better than regret. Even if you get turned down, it's not like they say "No I don't give my numbers out to losers" or something like that. Usually they'll say "I have a b/f" or "I don't give my number out to people I don't know". These girls usually aren't any fun anyway, usually got a stick up their @ss for whatever reason.
WTF Tookie? A promise ring? Something tells me you're not joking around either. LOL