Well, this is the most confusing and basic part of punctuation. Mostly new writers get confuse in this so let's clear our basic.

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Comma

Let's have a look on few basic rules of Comma and how to use them.

Do not use a comma just because your sentence is long. 

This is a common mistake; sometimes, people will see a long sentence and put in commas to “break it up” even if the sentence is grammatical without them. The length of your sentence has nothing to do with whether or not it needs commas.

If necessary, separate long sentences into two or more smaller sentences.

Use commas to offset expressions that interrupt the sentence.

Sometimes, prepositional phrases and other phrases interrupt the main sentence, serving as non-restrictive modifiers. If these interruptions are separate from the main subject and verb, separate the interruption phrase with commas to signal to the reader that this information is inessential.

For example, here is an example of a prepositional phrase acting as an interrupter:

“It is, in my opinion, an excellent book.”

This phrase isn’t crucial to the meaning, and can be taken out without damage to the sentence.

Here is another example:

“This road, on the other hand, is well-paved and easy to drive.”

Direct address also falls into this category.

For example, here is a sentence interrupted by a direct address to another person:

“That's why I'm appointing you, Thomas, leader of the group.”

Use FANBOYS to help you remember the coordinating conjunctions.

Coordinating conjunctions serve as connections within a sentence.

The coordinating conjunctions in English are For, And, Nor, But, Or, Yet, So.

Use commas before coordinating conjunctions that link independent clauses.

 An independent clause is part of a sentence that has its own subject and verb. It can stand on its own as a sentence. You should always use a comma when a FANBOYS conjunction connects two independent clauses.

Here is an example of a FANBOYS conjunction connecting two independent clauses:

“I checked out three books from the library, but now I find it impossible to read them all.”

If you removed the conjunction, each clause would stand as its own sentence.

This conjunction does not connect two independent clauses:

“Bai discovered he had everything he needed but a pencil.”

The last part of the sentence cannot stand on its own.

Pay attention to the grammar of the sentence. 

The presence of a conjunction does not necessarily require a comma. Only independent clauses need to have the comma between them.

For example, if your sentence joins only two words with a conjunction, do not use a comma:

“Give me all the bacon and eggs you have.”

If your sentence uses “for” as a preposition to join a dependent clause, do not use a comma:

“I’m saving money for a Hawaiian vacation.”

If your sentence uses “so” to emphasize another word, do not use a comma:

“The teacher was so tired of grading bad essays.”

If your sentence uses “so” in the phrase “so that,” do not use a comma:

“Elena knew that she should eat her breakfast so that she would not be hungry later.”

Use a comma after most introductory adverbs. 

Adverbs often end with the suffix -ly and modify verbs or adjectives. Adverbs are sometimes used at the beginning of sentences to explain how something happened or felt, especially in informal writing.

Common examples include generally, usually, and fortunately.

For example, here is a sentence that begins with an introductory adverb:

“Unsurprisingly, the one day I forgot my umbrella was the day it rained.”

Adverbs such as when and while usually introduce a restrictive modifier and should usually not be set off by commas.

Commas should be used to offset introductory adverbs that modify the entire sentence, not ones that modify a single element of the sentence (such as the verb).

Use a comma after introductory words. 
One-word introductory elements include words such as no, yes,and well, and are separated by a comma from the main clause when they begin a sentence.
Here is an example of an introductory word beginning a sentence:

“No, I cannot come over this morning.”

Here is an example of well as an introductory word: “Well, I’d love a second piece of cake, but I’m on a diet.”

Why can also be used as an introductory word, but be careful: it should only be set off by a comma when it is not essential to the meaning of the sentence.

The comma in the sentence “Why, that's amazing!” is correct.

However, you would not use a comma in this sentence:

“Why couldn’t you come over this morning?"

Use a comma to separate an absolute phrase. 

An absolute phrase, also called a nominative absolute, modifies the entire sentence. While they usually come before the rest of the sentence, they may also come after the main clause. An absolute phrase usually has its own subject and is generally formed with a "noun" and a "participle" (an "-ing" or "-ed" word).

Here is an example of an absolute phrase beginning a sentence:

“Her homework completed, Sujata left to meet her friends.”

Here is an example of an absolute phrase concluding a sentence:

“The couple hurried home, the cold nipping at their faces.” This phrase modifies the entire main clause before it.

Use commas to separate things in a list or series

 If you have a series of three or more items, use commas to separate each item.

For example, here is a list separated by commas:

“At the store I will buy apples,oranges, pears, and bananas.”✔

Do not place commas before or after the list or series. This comma usage is incorrect, as shown in this example:

“At the store I will buy, apples, oranges, pears, and bananas, to make fruit salad tonight.” ❌

Do not use commas when all the items in a series are linked by and, or, or nor.

For example, here is a list of items linked by and:
“Kyle and Spike and Brenda and Willow all went to the concert.”

If all of the items in your series are phrases instead of single words, or if the items in your list contain commas, it is acceptable to use semicolons to separate the items instead of commas:

“You have a choice of two breakfasts: granola, orange juice, and coffee,which is fairly cheap; or bacon, sausage, and eggs, which is more expensive."

Use a comma to separate dates and addresses. 

Each of the elements in a date (weekday, month and day, and year) should be separated by a comma. Also use a comma to separate the elements in an address, or when referring to a city and state or country.

For example, you would use a comma in this sentence: “I love to visit Tokyo, Japan.”

Here is an example of correct comma usage for a date:

“This Book was written on Monday, May 14th, 2007, in Maryland.”

When only the month and year appear, do NOT use a comma: “I wrote this article sometime in May 2007.”

Here is an example of correct comma usage for an address:

“Her new address is 1234 Main Street, Anytown, Maryland, 12345.”

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