Before you plan to ask a girl out, you first need to learn her well and know her emotions. This will prepare you to know how to deal with her and to what limit of words to use on her. Create enough space for mutual friendship with her, so you find it easy to give her your impression. Ask her out yourself. If you have decided to ask her out, you should do it personally. Get additional tips from http://www.ehow.com/how_5963658_ask-out-middle-school-successfully.html.
be polite, give her compliments, tell her why you like her and show her that shes the one. make a good impression that you are not a player or cheater and that you are worth it :) and lastly be yourslef and if she says no, then maybe shes not the one :)