You can have a look at a few of these as a guide:

http://www.activist.ca/guide/encrypt.html
http://searchsecurity.techtarget.com...212062,00.html
http://serendipity.magnet.ch/hermetic/crypto/intro.htm
http://www.itsecurity.com/papers/upaq.htm => a good intro
http://online.securityfocus.com/infocus/1181

Cheers,
Greg