The easiest way is the same as for other topics: Search for an open source project where you can help. They often have a list of tasks, sometimes a list of jobs to be done in different categories, from translating their website over artwork to C++ programming or whatever they are using.

Another way is to create something that helps you. Maybe you want to check the blockchain size from your smartphone? Write some PHP program which checks the size of the file on your PC. Maybe you start reading about the format of the file and extract more interesting data and finally you created some useful project for others. I do not know what interests you and what you can do, you need to find for yourself what you want to do.

For the knowledge part: There is documentation about the program, the protocol and BIP-documents for new (and existing) features. The client itself is open source and miners are as well. You will need to read some more technical documents or source code, but as a developer this will probably be not new for you.