Running out of Hard Drive Space

Hi there my name is Mark, I am not very good at technical things so please bear with me. I have a Macbook Air, I have only used it for storing photos from my holiday and I have this message coming up saying Start Up Disk Full. I moved all my pictures to a external hard drive but could not move my video's. I only have about 10GB off video material and have moved allot to Facebook just to save them. Now my Mac says the hard drive is full and i can't understand why. I go into files and there is nothing large there at all, my trash is empty and i cannot see why or what is causing my Mac to be full. Please help I am cracking up fast.

I have attached a picture.