With your mouth.... Just kidding. Im not a pro but the way you need to ask a girl out needs to be personal, not with your friends there because they give you more pressure. You also need to tell her straight up, not in texting. You should give her little romantic details, or surprise her with something. and if you want just be simple. Good luck!!
Be yourself. Do not ask your friend to ask her out for you because honestly i think that's dumb... Im 13 too and im a girl i would love if someone asked me out with a rose or something cute c: hope this helped