Disc full message, what to do?

Please give us the last three letters of your serial number so we know what iMac you have. You have most likely downloaded to many movies and/or music. You need about 10% free hard drive space for your machine to operate properly. Drag duplicate files to the trash and empty the trash to get rid of them. You may also have duplicates in iTunes, to fix this open iTunes and open the "show duplicates" option and start deleting them. Then you can do what Frank suggested and get a large external hard drive for storage and Time Machine backup. You can also install a larger internal hard drive. We can give you directions on that after we know which machine you have. Operated with a full hard drive will introduce errors into your data. I would advise defragmenting your files and optimizing your volume after you have enough free space. Use something like TechTools Pro to do this.