Hard Drive is Full - Please help

I have an older MacBook Pro (Early 2011- 15")still running on Sierra OS. I know it will run slow but it is running much slower.

Right before my wife passed almost two years ago I was copying files from a hard disk to the built in CD Rom. Well the Hard Driver ran out of room because it filled up with the temp files it wrote to write it to the CD. The program froze and I had to shut down. I was never able to complete the writing of the files on the CD.

I am guessing that the files are on the Hard Drive somewhere because my hard drive is full and was never even close to full before I attempted to write the files to a CD.

If correct, where will I find those file where I can delete them and at least be able to use my old machine for other purposes like for the Grandkids when they come over!

Thank you for your help