Well, as a general rule, start with the man pages. man tar.
You could google around, but that can be overwhelming when the topic has too many general keywords. I found http://www.linuxselfhelp.com/gnu/tar..._9.html#SEC128
which gives some examples with multiple archives on a tape.
The one difference between their examples and what I usually do is that I typically use the v option for verbose. To me it is somewhat comforting to see the terminal scrolling up rapidly so that I know it is doing stuff.
If you get comfortable with that, have lots of computers to do, and find that it's getting to be too much manual labor, then you could always adopt amanda http://www.linuxquestions.org/bookmarks/tags/amanda
and let it manage and schedule your gnutar, gzip and tapes. But that's probably overkill if you are just learning and just doing your own computer. On the other hand, if you are pushing yourself to learn more, you might. ;-)