How do you tell a girl for sex?

Be more specific with your questions in the future, cause it could be different if this 'girl' is your girlfriend, or just some girl you've been seeing for the past couple of days/weeks/months.

Anyway, I assume it is your girlfriend, I suggest you 'love' this girl before you do anything, because.. even in your benefit sex is better if you love what your making love to, and if you love your girlfriend it would be easier to confront her because.. you know her, it's not just a ' How do you do it ' sort of question it's not a game. That's all I can say. And if my advice does work DO NOT, I REPEAT DO NOT FORGET A CONDOM. you don't want little mini-yous running around, they're annoying, expensive and.. they break stuff. so DON'T FORGET THAT CONDOM.

Hope my advice helped :)