Desperate help with a girl?

I'm 14, a freshman in high school, and i like this girl. she's not in any of my cles.i talk to her on myspace but i never see her in my cles i see her in the halls ocasionaly but thats it. i just sen her a message 4 days ago saying sup and she went off w/ the "OMG, i havent talked to u in so long, i havent seen u at school, wat up." i cant tell if she likes me or not and regardless i just need to know how long to wait to ask her out and then a way to ask her out. mybackyard just got finished and i was going to ask her if she wanted to come over and swim or play tennis at my house. but i want to ask her to a movie or something after that. please help