Find a background tiling image from this site:
http://background-tiles.com
Then click on "Show Me More" for Patterns, Textures or a Color
When you find one you might like, first Preview it and then Download the file.
Once you download it, go to the bottom-left of your Chrome window and Show in Folder.
Then copy the file. Navigate to your website folder and paste it in there.
As you do the next step, keep in mind that you can easily see what the file name and type of your picture is by looking at the bottom-left of your Chrome window...
Then in your style sheet, go into body and add an empty line...
Save your style sheet and take a look at any of your webpages in Chrome.
If you like it, go to the next page of instructions using the button below
If not, try out some more pictures; I found this grey one I liked.
If you've downloaded and tried out a few different pictures, make sure to clean up your website folder, deleting any background images you won't be using.