Startup disk almost full

This is my husband's iMac. It has become all but unusable. Every click of the mouse results in 3-15 seconds of waiting for the spinning wheel. I transferred the amount you see from my EHD to the Mac a few years ago planning to make a bunch of home movies. Add all of our apple device new video captures and I guess over half the storage is now video files--which I am guessing is causing the computer to run so slowly. The problem is iMovie doesn't have enough disk space to make NEW movies. So how do I delete what isn't important and how much space needs to be cleared so I can use iMovie to get rid of all the video files? It's a vicious circle... :/ Also, I have completed video projects that I can delete, but I don't know if it matters if I do? They are saved in iTunes. I think I learned from a previous conversation with Apple tech support that files are duplicated and can be in triplicate on your hard drive. Thanks in advance for any help!