Note
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The first thing you should probably do is to browse the CGNS documentation to see if CGNS is right for your application. There are also a number of conference papers and slide presentations available which you may want to read. You will find it highly beneficial to join the CGNS forum on github. This forum has been set up to facilitate the exchange of information among CGNS users. This is a good source for answering any questions you might have.
Next, download the free software sources from github. In order to build the library you will need an ANSI compliant C99 compiler. You will also need cmake to setup the compiler and options. The full installation procedure is detailed here. Note that there are some test cases in the tests subdirectory, that you may also want to build and run.
Now, to building your own application. If you intend to use C and/or Fortran, then you are ready to go. Create your program, and link with the CGNS library. The Guide to CGNS is useful for the creation of your first program.
For developing in C++, you may want to call the CGNS library routines from within your C++ code. The CGNS library source in itself is C, and as such does not provide an object-orientated interface.
If you prefer working in Python, then check out pyCGNS, which is a Python CGNS API.
Finally, a number of utilities have been developed for CGNS files, which you may find useful (like cgnsview to examine CGNS files).