How do I make a complte back up of my airbook catalina and restore it to original

If, as you say, your computer is quitting because it has no storage left, you do indeed have a serious problem. Unless corrected, your computer can stop functioning. To make a backup you need a dedicated external disk. Then use the Time Machine app to do regular backups. However, that will not solve your problem. What you need to do is to clear data off of your hard drive. You can do that by either deleting unneeded data (files) or install a second external disk. Then copy files onto it so they can then be deleted from the HD to clear needed space. In summary, you need two external disks, one for backing up and one to move existing files onto. That is what will solve your problems.