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JCL isn't really a programming language, it is a method to tell z/OS which program to run, using the specified input and output. You can also use it to tell the system which steps to skip given certain return codes.
A good starting point would be to produce your own EREP for the system, this is an event and error report and, in producing it, it gives you a very basic idea of how JCL is structured (best to check with your system programmer that this is ok to do) -
Here is a beginner's guide to JCL (it asks that you know COBOL as a pre-req, not sure that is 100% necessary) -