You dont say what your computer work is, it may be one Word file, or hundreds of files for a major project. Backing up one file is of course a lot easier than backing up hundreds of files.
But in simple terms a "backup" is just a spare copy.
The very simplest way to create a backup is to create a new folder, called say "backup", and to COPY (not move) the file into this backup folder so you have a spare in case the original is lost.
Of course every time you change the original file you should always create a backup, so when you copy the original file into the backup folder you can add 001 to the end of the file name, then 002, then 003 and so on.
That way you have a whole set of backups and can always go back to a previous version of the file.
However having a "spare copy" on the same machine as your original is not a great idea in case the original computer crashes or is lost or stolen.
So it is a good idea to buy a memory stick and / or an external hard drive.
Then you can make copies of your original file onto both the memory stick AND your external hard dirve.
The important thing to remember with backups is you have to think of "worse case" scenario.
If my laptop was stolen would I lose all my work or would I have a recent copy of the work to go back to.
So you should ALWAYS have a recent copy of your work on ANOTHER medium (be it memory stick, external hard drive, even CD or DVD or another computer).
You can NEVER have too many backups.