When you see a girl that you like you should smile and speak to her. If she reacts in kind you can start up a conversation with her asking her about things she might like such as what her favorite movie is. After you have established a friendship ask her if you can call her and text her and take it from there.
Thirteen is a little young to have a girlfriend; you should definitely make sure her parents are ok with it. But the easiest way to get a girlfriend is to find a girl that you like and ask her if she’d like to go out with you.
Talk to her and get to know what she likes to do and is interested in. Spend some time with her and invite her to go along with you on a group outing. If you feel that she likes you, go ahead and ask her for a date after you clear it with your parents and arrange for transportation.