First of all, we tell the assembler what kind of output we expect, which is the GUI executable (although, it would be without any GUI), what our entrypoint is, and, of course, we include the win32a.inc file in order to be able to call the ExitProcess() Windows API. Then, we create the code section:

format PE GUI                             ; Specify output file format
entry _start                              ; Specify entry point
include 'win32a.inc'                      ; Include some macros
section '.text' code readable executable  ; Start code section