A quick insight into the F77 standard can be found online at
http://www.chem.ox.ac.uk/fortran/fortran1.htmlThis site contains the most useful types of variables, loops and input/output operations you might need to use in your codes. If you seek a more complete documentation, please look at
http://www.univ-orleans.fr/EXT/ASTEX/astex/doc/en/prof77/html/prof77.htmwhich provides almost complete description of the F77 standard. There are also many useful books on Fortran, many of them devoted already to a more recent version, Fortran 90. Once you beat F77 and decide to learn Fortran in more detail, I suggest to check with an excellent book of Ellis, Phillips, and Lahey: Fortran 90 programming, Addison Wesley, 1994.