a.m., p.m.
Lowercase, periods, no space.
Use figures with these terms:
3 a.m. 4 p.m.
but three o'clock
10 o'clock
the 4 a.m. bell (no hyphen)
the 4 p.m. bell (no hyphen)
Write noon or midnight, not 12 a.m. or 12 p.m.
See also midnight, noon, TIME OF DAY.