How do you make include files?

[slashdot]

I have noticed that it is increasingly difficult to continue further in my game I am making, unless I have a way to disperse the clutter I have in my one file. How exactly do you make an include file? Where should it be put at? Do I have to link it when I do g++ file.cpp -o file? (Like done with SDL) Do I name it a certain something? Thanx in advance!

[redhead]

Include files are usualy named .h which is the header files that you use in order to tell other parts of your program what functions are available and how to use them.
I havn't got time right now to fully explain it, but later today I will describe it more specific and give some examples, for now the one thread that comes to mind, would be this one with my last post extending it to the fullest.
Basicaly you make a matching header (.h) file to predefine the functions available for everyone to use, then place the header aswell as the .c/.cpp file in the same folder as the project, include the header file by using " to surround it instead of <> precompile the provided .cpp file following the header into a .o file, then precompile your end program into a .o file, and link them all together into the executable.
It helps if you fully understand the preprocessor and what it does.