Even with a library like BouncyGPG it is impossible to correctly use the cryptography toolbox without a bit of theoretical background. The HOWTOs will give solid implementation guidelines regarding selected use cases.
The provide example code can act as starting point for your implementations.
Reading the use cases will provide you with enough information to implement them for “normal security” applications. For high security applications, e.g. when the life of people is at stake involve a cryptographer!
Cryptographic keys are the input parameters to many cryptographic operations, namely en-/decryption and signatures. This HOWTO will show you how to create keys from passwords, and how to derive multiple keys from one master key.