The main sources of FITS information are described at a number of web sites, in particular the FITS Support Office of the NASA Goddard Space Flight Center (http://fits.gsfc.nasa.gov). Other useful sites are the NASA High Energy Astrophysics Science Archive Research Center (http://legacy.gsfc.nasa.gov/docs/heasarc/fits.html) and the NRAO FITS page (http://www.cv.nrao.edu/fits/).
The definition of FITS is given by by the NASA/Science Office of Standards and Technology standard NOST 100-2.0 (http://fits.gsfc.nasa.gov/documents.html). A (somewhat dated) User Guide is available from the same location.
Apart from the formal standard, many of the following recommendations are based on the HEASARC FITS Format recommendations (http://legacy.gsfc.nasa.gov/docs/heasarc/ofwg/ofwg_recomm.html)
Links to a number of FITS keyword dictionaries are available from the FITS Coordinating group at http://legacy.gsfc.nasa.gov/fcg). The choice of keywords is described in more detail in section 2.1.
The Single Dish FITS convention is described in AIPS++ Note 236.
Links to a number of FITS-related software libraries and tools are available from the HEASARC web page. In particular, we recommend using FITSIO for creating FITS files, and the FTOOLS package (in particular fv, what else?) for inspecting and analyzing FITS files.