iTunes cant locate any of my music!

i dont know where to find any of the music!! im kinda new to this so i dont know much.. i always had someone else do it for me. anyways! when i try to listen to a song on iTunes a thing pops up saying "The song ____ could not be used because of the origanal file could not be found. whould you like to locate it?" & i have nooo idea what to doo! pleease pleasee help me! music is my life! D: