Itunes question plzz answer :-(?

i was on my moms username on mii computer and i redeemed an itunes gift card thenn i boughtt boom boom pow and tried to put it on mii itouch but it said i couldnt cuz itt would delete all of mii songs and apps then i went to put go on mii username but it said i didnt have any money. Does anyone no how to gett the money on 2 mii acountt??? plzz help mee. If this helps, 2mrws mii birthdayy and thats y i have the gift cards. lol plzz help thnxx