basic problem that if solved will contribute to my happiness

i have a very limited hard drive disc space, only 7 gigs, all of which has been taken up by my mp3. Now I do not want to buy any more memory, instead i'd like to blank my itunes library without it automatically blanking my ipod nano when i attach it to charge. how can i do this so that i can add tunes to the existing ones on my nano whilst starting from a fresh itunes library. please help.