The comma is a punctuation mark that's frequently used. Take note of the following tips as they will be useful for the writing module of the IELTS.
A comma is used:
1) between a list of words
Example: North, South, East and West.
2) before a conjunction
Example: I went to school, but I was late.
3) to indicate contrast
Example: The cat was brown, not gray, and it was big.
4) in instances where a phrase could be separated with brackets
The car, which I drove, is really fast.
5) in instances where a phrase adds related information.
Example: Doris, the owner of the shop, is easy to approach.
6) in instances where the main clause of the sentence is dependent on the clause before it.
Example: Though I woke up early, I still got caught in traffic.
7) in introductory phrases.
Example: As a rule, only registered members could use the facilities.
8) when there are conjunctive verbs.
Example: Furthermore, he possesses the necessary skills for the task.
9) in instances where you use "for example"(e.g.), "that is" (i.e.), etc.
Example: I'm going to the place where I work best, i.e., the coffee shop.
10) in instances where a pause is required
Example: Whatever happens, just stay calm.
11) to avoid confusion
Example: To Paolo, Jenny gave her last candy.
12) when you address someone by name
Example: So, Genevieve, I'm taking you to the movies.
13) when you quote a direct speech
Example: Then he said, "Please Sir, May I have some more?"
14)when indicating the omission of a word or phrase
Example: Put too much water and the mixture will be weak, [put] too little and it will be strong.
Here's a video by Paige Carrera further discussing how to properly use commas.
Attribution to Rebecca Elliot for her book Painless Grammar and to Paige Carrerra
Other punctuation marks:
Parentheses, Brackets, or Dashes