Have a good ante meridiem!
a.m. is the abbreviation for the Latin phrase ante meridiem which means before noon.
p.m. is the abbreviation for the Latin phrase post meridiem which means after noon.
11:00 p.m. is one hour before midnight
11:59 p.m. is one minute before midnight
12:00 midnight (even if your digital clock shows 12:00 a.m.)
12:01 a.m. is a minute past midnight
1:00 a.m. is an hour past midnight
11:00 a.m. is one hour before noon
11:59 a.m. is one minute before noon
12:00 p.m. noon (even if your digital clock shows 12:00 p.m.)
12:01 p.m. is a minute past noon
1:00 p.m. is an hour past noon
There is no need to say morning, afternoon, evening or night after using a.m. or p.m.
Incorrect: The meeting will be at 11:30 a.m. in the morning.
Correct: The meeting will be at 11:30 a.m.
Correct: The meeting will be at 11:30 in the morning.
Note: When a sentence ends with an abbreviation you do not need another period.
One space between sentences.
On a typewriter you might have been taught to place two spaces after a sentence. That does not apply to typing on a computer. Since computers use proportional fonts, where the space taken by each character varies, two spaces can leave a disturbing gap. This also applies to all other punctuation marks.