start up disk, need help please

my start up disk tells me i am running out of space, i have a mac book pro, 80gb disk space. My problem lies with i tunes and i need to know can you increase the space on i tunes to put more tunes in it at the moment i have about 30/40 albums in it and am running 1 program ableton live 6. I was told this mac would be over kill for just one program. my 4gb phone holds about 20 albums mp3 128, so please can you tell me how to accesses the hard drives space.....many thanks pepy