Its basically a matter of what they are used for. If you want to e.g. create something new or upload a file you shoulshould use a POST request. If you want to get Information from the Server, which is already there (e.g. data from a database) you should use GET.
To sum it up in short: Use POST for sending data and GET to receive data from the server.
For your form: you have to specify which request method should be used:
<html> // Use GET <form action="form.php" method="GET"> <input type="text" name="text"> <button type="submit">Submit</button> </form> // Use POST <form action="form.php" method="POST"> <input type="text" name="text"> <button type="submit">Submit</button> </form> </html>