Answer:
1. Write the program you want to write.
2. Compile the program. To do this, type f77 xxxxx.f into the command line. If this gives an error, you probably haven't installed a Fortran compiler like for example gfortran yet.
3. Run the program. The compiler has produced a file called a.out. Run this file by typing ./a.out.
4. Understand what you just wrote.
5. Add your comment. To add a comment that has an entire line on its own, write a c directly into a new line, without the 6 spaces. After that, write your comment. You should leave a space between the c and your comment for better readability, but this isn't required. Note that you have to use a ! instead of a c in Fortran 95 and newer.
To add a comment in the same line as code, add a ! where you want your comment to begin. Again, a space isn't required, but improves readability.
