Is it legal to put a song in the background of a video you edit?

okay so i'm looking to edit some camp films for people and picture montages. anyway, i'm wondering if its legal to use a song in the background from itunes or a cd. i've put some songs in the background for my church and friends before but i don't want to do it for strangers without checking on the legality of it first. if i have to obtain a license to use the songs what steps do i need to take to do that?