From Lunarpages Web Hosting Wiki
This tutorial will show you how to manage your databases from inside of Plesk. Databases are a very handy way to store information you would like to have displayed on your web site. If your web site is the plate for your dinner, the databases would be the meat and potatoes.
How to Create a Database in Plesk
The first thing you will need to do is login to Plesk and get yourself to the front page. Once you have made yourself there, click the domain name you are going to create the database for.
Create a Database
Now, click on the icon labeled "Databases". On the next screen, you can create a new one by going to "Add New Database".
Fill in the empty text box with the name you wish to use for the database. Next, select from the drop down menu what type of database you wish to create. Once you have done both of these things, click on the "OK" button to create your database.
Create a User for the Database
You have the database created, so now you need a user for that database. Navigate to the page for the database you just created (under Plesk > Database) and click on "Add New Database User".
Create a unique user name, password, then confirm your password and hit the "OK" button. You have now created a database and a user for that database.