Start up disk is almost full. How do I fix this?

How do I fix my start up disk. I have a mac mini given to me from my Uncle, I have only had it this year. I was a Dell user and that was a bummer. I understand that I need to look under app/utilities. But How do I fix this without messing up my mac.

No, you start by emptying the Finder Trash,
Then you go into iPhoto and Empty the iPhoto Trash,

then you find and backup, then Delete any Movies you have.

Hard drives have a fixed amount of space when you full it up then it's full and you need to remove so files.