The simple rule is that you should use commas any time you are adding additional information into a sentence.

Introductory phrase:
If you add words onto the front of a sentence, add a comma to separate the extra information from the main sentence.

Generally, football players need to work out all year long.
In fact, football players need to work out all year long.
Unlike other athletes, football players need to work out all year long.
No matter what you think, football players need to work out all year long.

Interrupters and Interjections:
If you add a phrase or word in the middle of the sentence, put a comma on either side of the extra information (unless they're at the end in which case you just put the comma in front of it),

Football players, in fact, need to work out all year long.
Football players need to work out all year long, you know.
Football players, go football, need to work out all year long.

Appositives:
If you stop and define or explain a noun in the middle of the sentence, you need to set it off from the rest of the sentence

Football players, both professional and student athletes, need to work out all year long.
Football players, or soccer players as the Americans call them, need to work out all year long.

Place commas where they are needed

April in the meantime went swimming.

Okay maybe it's not so bad.

We visited the home of Harriet Beecher Stowe the author of Uncle Tom's Cabin.

There's little chance in my opinion that the war will end soon.

The firm chose Mary vice president of public affairs as its chief executive officer.

I live in New Mexico the "land of enchantment."

Neil Armstrong the first man who walked on the moon is a native of Ohio.