How do I find out where my memory is distributed?

Hi,

I am having some power issues and am in the process of sending my mac in for repairs so I am backing it all up onto a Seagate Backup Plus.

In the process of doing this I was trying to consoliate my itunes library onto my HD and it said I only have 10GB left availble. How can I see what is taking up the most room so I can start deleting ect?

Also, I am using TimeMachine to back up all my files so will it automatically back up my music even if its not all on my HD (I can only find most of it in Itunes)

Thanks!