First let me wish you luck on such an ambitious quest....second, definately read Bruce Schneider's book as recommended above.

However, what you are trying to do is not a code thing, it is a math thing and you are going to want to check out sites devoted to cryptography as you will first need to develop a cryptographic formula that you can then implement in code.

A great starting point is Matt Blaze's Cryptography Resource.

I would also suggest the AES Home Page for an example since is appears that this will be the new 'standard'

If you are just starting to get into this in a real way, you will want to peruse the RSA Crypto FAQ as it has links and information on just about everything related.

And lastly, just because I thought it was cool reading I will include Courterpane Labs Blowfish page

Good luck, here is to hoping you become the next Whitfield Diffie or Martin Helman - ;-)