How to ask girl to dance?

There is a valentines day dance at my school and I did not find out until today. And there is this girl that I like who is really pretty and nice to me and yet I get nervouse around her but still talk to her she is in two of my classes. now I want to ask her too it but I will get nervouse and end up not going with her how can I ask her without being embarrassed around her friends if she says no. Also how do I bring it up the conversation to her and how to start off asking her and how to end it.

Also other guys think she is hot but don’t actually like her and she is popular but not a snob so I got some completion between me and guys that completely like her for looks. When should I ask her but also not be too late. Add any more information you think is needed

Also how do I get her alone to ask her we do stay after one day a week
Will she reject me if I’m not popular and if she is popular

2nd question

Is it ok to ask another girl on Facebook about the girl that I like
This girl that I am going to ask know her better so you know get some advice from her
The girl that I am getting advice from doesnt know I like this other girl and I am afraid to tell her because we all know Girls tell their friends a lot of things and talk

So I’d it a good idea to get advice from the girl that I like friends? Or not