Fortran eh? Why that language?
Also, if you have the option to use anything but Windows instead of Windows for this, I think I'd recommend that.
(1) you write the code as text wherever I suppose (in a brief example of calculating pi with monte carlo i.e. random simulation method it was just in notepad) and save it as ".f90" file extension or whatever is appropriate (whether it's an old fortran 77 file, a free application, etc; this is described in the user manual)
Yes. I would recommend not to use the default notepad, but some editor more specifically tuned to programming (with syntax highlighting), e.g. Notepad++ for Windows.
(2) you go to the right directories/etc in the cmd prompt and compile the code in some manner. You get the ".f90" file and compile it into a .exe with the "[filenames] -o [futureFileName.exe]" command; however, there are -x language to compile into another language (C or C++), but what are the -c ("Compile and assemble, but do not link), -S ("Compile only), -E (Preprocess but don't compile), etc. Basically what does it mean to "assemble" or "link" for compiling code
Compiling turns your code from something you can read to something the computer can execute. This happens in several steps. For bigger projects, it is often useful to divide your code into different files and you might even want to use your file a.f90 for different projects. It is possible to turn your code into something which is not executable itself, but which is already written more towards machine code, so that other files can use it (without having to compile it at that moment).
I'd recommend you to read a good manual on this when you get to the level where you need this.
http://www.g95.org/docs.shtml gives some examples in its basic options.
(3) you run the newly compiled file in the right language (the .exe program) from the cmd prompt. However, how do you debug code or find errors? Java will tell you "error line 121" when you compile, Matlab will also tell you "error line 121" or whatever when it runs or you can put a break in the function and go through everything step-by-step, but this FORTRAN compiler just spits out a .exe you run.
Code might not compile, giving you an error message about what went wrong (and line number). Code might also crash during runtime, giving you an error message (hopefully with some useful info).
A break is not generally possible, but it is possible to spew out information about the state of the system prior to the crash manually.
How can you find errors in your code?
A nice mix of experience in decoding error messages, google and black magic.
I recommend you to use the -Wall compile option, it will warn you when you do things which are not 100% kosher but might work, it is usually better to fix these things (or at least understand them), since they might lead to unexpected results otherwise.
