As taken from The WordPress MU Manual
Before you can install WPMU you first need to create a MySQL database and user on your domain using your cPanel.
In simple terms your MySQL database stores all blog information including files, blog posts and comments and is backed-up almost constantly by your hosting company to ensure no data is lost.
1. Log into cPanel using your account details supplied by your hosting company.
NOTE: Appearance of cPanel does vary slightly depending on what theme your hosting company uses.
2. Scroll down to the Database section of cPanel and click on MySQL Databases
3. Create a database by entering the name WPMU and clicking create database
4. Once the database has been created click on the Go Back link.
Note: the name of your database and user are both given the default prefix of your domain name. This is to ensure it has a unique name from other databases when it is stored on your host’s servers.
5. Next create a new user by adding their username and password then click Create User.
6. Once the user has been created click on the Go Back link
7. Now add your user to your database by scrolling down to Add User to Database and click on Add.
8. On the Manage User Privileges page select All Privileges.
See Next: Installing WordPress MU
