Depends on the girl. Do you know her? Does she know you? Is she a friend already? What signals of interest have you had?
The bottom line is that if she is interested she will say yes no matter how you ask her, provided you aren't a total jerk. Just be polite and suggest going out.
If she says no, don't worry about it, find another girl who says yes. It's always nicer to date someone that is as into you as you are into them, so don't sweat the no's. At the same time she might say yes, but you never know until you try.
Don't make such a big deal out of it and it will be fine either way. You don't need help. Just go for it.