There are three macros that make section definition as simple as possible: CODESEG, DATASEG and UDATASEG (similar to tasm ideal mode syntax). END macro marks end of file.
A program should have at least CODESEG (.text) section and END macro, other sections are optional. CODESEG is read-only, DATASEG and UDATASEG are read-write; i.e. you can place data in CODESEG as long as you do not change it. You can also define your own sections if you want, but there's very rare need to do so. Each section (even if it is empty) enlarges your executable.
START macro tells linker the entry point, and must be present (if you are not making a library).
Thus, a program's skeleton should look like:
%include "system.inc" CODESEG START: ;entry point ;code DATASEG ;data UDATASEG ;bss END