To put it briefly, you use the POST method when your request is meant to change the state of the server. That would include updating databases, adding to shopping carts, and things like that. You use the GET method when your request is not meant to change the server's state; that would include requests for information from the server.
There are grey areas and complexities which I haven't mentioned, as there always are in questions of this type, but that's the general idea.
For example if my request is supposed to make a dentist appointment for me next Tuesday at 11 AM, that would be a POST request because that information is going to be stored at the server. However if my request is supposed to find out what times are available for appointments on Thursday, that would be a GET request.
We use POST method when we want to submit the data to the server, For example if you are submitting the details of an employee. Use GET method when you want to bookmark your page or retrieve simple data. GET is not secure since the data is appended in the URL, in POST data is sent in the body so the data is not seen to outside world.