Schneier's Applied Cryptography is a very readable introduction.
If you want to get into the actual mathematics of it, then it is considerably more difficult. Luckily for me I took courses in pure math, RSA and some hashing functions at university, so I had to learn it :-)
As others have said, crypto is properly hard and it's very easy to make cock-ups. So unless you're prepared to do the years of work becoming a proper researcher, don't try and design any algorithms yourself (for fun is ok, but not for real world use).