Some people use the salt shaker approach to commas: sprinkle them about, anywhere they think there might be a pause, and hope, that they are useful. Some people listen for commas. Neither of these is the correct way to use commas. These approaches only work in a few instances, and when they do, there is always a grammatical rule to back it up. (Note that the commas in the first sentence were intentionally used incorrectly.)
A comma is often used after an introductory element at the beginning of a sentence. These elements can include introductory interjections, prepositional phrases, absolute phrases, adverbs, and transitional expressions.
Use a comma after introductory interjections that don't display a lot of emotion.
Yes, we should go soon.
No, I didn't bring any glue.
Well, I'm not sure that's a good idea.
Ugh, this backpack is heavy.
Introductory Adverb Prepositional Phrases
Use a comma after an introductory adverb prepositional phrase unless it is very short (3-4 words) and is not likely to be misunderstood.
Between the towering city buildings, the sky could be seen.
Use a comma because the introductory prepositional phrase is more than four words.
Before eating, Lauren put the pots in the sink to soak.
Use a comma to clarify that no one is eating Lauren.
In the morning we have to get up really early.
No comma is necessary because the introductory phrase is only three words.
Use a comma after an absolute phrase at the beginning, in the middle, or at the end of the sentence.
The rain finally over, we left the shelter of the trees and walked on.
The puppy, its eyes finally closed, slept the rest of the car ride home.
Generally, use a comma after an adverb at the beginning of the sentence.
Quietly, we tiptoed past the bedroom door so that we wouldn't wake Dad.
Introductory Transitional Expressions
Use a comma after an introductory transitional expression, like a conjunctive adverb (however, therefore, moreover, etc.) or a transitional phrase (for example, as a matter of fact, in other words, etc.).
Next, we will analyze Shakespeare's play Twelfth Night.
In conclusion, I believe that the school should require uniforms.