What is the best strategy for cleaning some of the data from an Imac?

I have a fairly new Imac that I share with my 3 kids. It is running incredibly slowly because we never get rid of anything. What is the best way to start cleaning some of the unused data off?What is the best strategy for cleaning some of the data from an Imac?
Unless the hard disc is nearly full (unlikely with a fairly new iMac) never getting rid of anything will not be the problem causing the slowness.



What sort of things do you have on it? Movies will take up space (3 - 5 GB each).



Large external discs are relatively cheap and can store copious amounts of stuff leaving space on the Mac's internal hard disc.