HELP !!! I tried to defrag my computer and it says I have used 18.49 g?

I need to defrag my comp. I have used 18.49 g of space and only have 148mb of space left. Is there a (free) program available to empty some unneeded, old or unwanted files. I would concider buying one if it was reasonably priced, and dependable. I am the second owner and it had alot of crap on it when I bought it, Pictures, files etc. I see them pop up on occasion. PLEASE HELP..... TAG