There are plenty of reference materials about GnuCOBOL, the Free and Open Source Software implementation of COBOL. You can find pointers on the Resources page of the Camelian.Systems Web Site. A particularly good on-line reference the for COBOL language is at MaineFrameTechHelp. It was designed for their tutorials.
The Tutorial has the links to the tutorial pages. Each page has examples, discussion, and the expected output for each one. It is best to follow sequentially, each page need not take very long to go through.
For help installing GnuCOBOL on Linux, Windows, or Mac OS, see the Camelian.Systems Web site.