I assumed you have already create your database, if not, kindly click on this link to learn How to create a Database
From your cPanel, under the Databse Tab, click on MySQL Database
Scroll down to where you have MySQL Users (Add New User)
Enter your Database user name and password as indicated in the screenshot below and click on Create
After it has been created successfully, click on Go Back
You will be taken back to the MySQL Database page, then scroll down to where you have "Add user to Database" like the screenshot below
Select the database you created and the user, then click on "Add"
On the next page, click on "All Priviledges" and at the bottom, click on make changes, as shown the the screenshot below: