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.

Use the U.S. Postal Service format (two letters, no periods) when you must abbreviate state names. In running text, unless the copy is a lengthy list of place names, it’s preferable to spell out the state name, especially if your audience might include international readers. Note that the third example needs semicolons between entries because individual items in the list include commas.

• The class of 12 included students from Alaska, Colorado, Connecticut, Georgia, Montana, Minnesota, and West Virginia.
• Research was conducted in Mesa, Arizona, and Las Vegas, Nevada.
• Those responding to the call for special funds included alumni from Harrisburg, PA; Fargo, ND; Miami, FL; San Jose, CA; Midland, TX; Shreveport, LA; and Hope, AR.

From the University of Colorado at Boulder Style Guide

Use periods with the two-letter abbreviation for United States. Do not use periods with the three-letter abbreviation for United States of America. Use the abbreviation U.S. only as an adjective. Spell out United States when it is a noun.

• The U.S. government is one of the nation’s largest employers.
• Most of their products were made in the USA.

From the University of Colorado at Boulder Style Guide