Not enough space on computer (mac)?

I can't sync or back up my iphone 5 to my mac. I thought i deleted enough files and things. What do i need to do and how do i see how much space i have on my hard drive? How much space do i need to maintain on my mac so i won't have this problem again? I know i have 16 gb of music on my itunes…

"how do i see how much space i have on my hard drive?"
Select it in a Finder window, and press command I.

"what do i need to do"
See the "Best Answer" below for HDD is full.

"how much space do i need to maintain on my mac"
As much as possible. No one but you knows how much stuff you want to add to the computer.