How to connect a mobile application to a web service?

Question

Java

Good time of the day. My task is to write a web service that will be deployed on Heroku.
There is a mobile application that should make requests to a web service, and that, in turn, pull data from MySQL.
I have already googled and read a lot of things, but I did not come to a specific answer: how to make requests to a web service from a mobile application? GET and POST requests?
I just got into web development and this amount of information knocks me down. Please tell me which implementation will be the most correct or in which direction to google to find the answer. This topic interested me very much.
I write in Java.