Cryptanalysis is the art of breaking codes. If you are new to codes and codebreaking, then you probably want to read this introduction.
If you know a little bit about codes and codebreaking, and you want to learn more, then this is the place to start. You can read about some of the ciphers which are used to turn plaintext into ciphertext: how to use them, how to recognise them, and how to break them.
It is also the place to look if you want information about a particular cipher.
Perhaps you have got some cryptograms that you would like to decipher. You can use these codebreaking tools to help you.
Past cipher challenges provide cryptograms for you to decipher.
Frequency analysis can be used to break many ciphers.
If you are serious about cryptanalysis, then you will want to learn about computer programming.