There are usually five kinds of information given in an assembler file. The exact syntax depends on the processor and the compiler, so you should use the compiler 'cc -S' to tell you which form to use. Most assemblers only accept input files that are in the same format that a compiler would produce. Be careful to notice how tabs and spaces are used.
Some assemblers use the C preprocessor.
A directive is your only way to communicate with the assembler about what it should do with the following lines of input. The most important directives are the ones that switch between the data and text sections. Undefined symbols are usually considered to be external. Defined symbols are usually considered to be local, and you will need a directive to make the name known globally. The best approach to learning more about a computer system is to take it apart and look at the guts.