I learnt about those a while back from some book or other - pretty simple, but very interesting. Another way is to use a keyword to encrypt it:
1 2 3 4 5
1 S N A K E
2 B C D F G
3 H I J L M
4 O P Q R T
5 U V W Y Z
(we use consider 'X' to be the same as 'Z'). You then take the numbers (L = 34, for example) and output them. When you've finished this project, perhaps that'll interest you further.
On a side note, I really must get around to reading The Da Vinci Code - reading Atlas Shrugged at the mo.