What do i say when a guy asks me out?

ok...i have never had a boyfriend before and im 15 -___- okk, soo this guy i like ALLOTT is planning to ask me out, i heard. i know your thinking rumors...but you never know. if he says "will you go out w| me?" i dont know what to say! i dont want to get nervous! but i like him soo much! <3 how do i say yes? im not sure? :( HELP! il rate! :)