USERNAME = The user name use with your server
PASSWORD = Again the same as with your server
HOST = Your Server
Example:
If you set up a database locally on your computer.
USERNAME would be = root (your administrator name)
PASSWORD would be = ********** (your administrator password)
HOST would be = localhost (As my im using my local server)
Otherwise - your server has changed these details when setting up your database. You need to ask them what they changed them too.
Note: As a PHPmyAdmin is sometimes not defulting a password so you might have none. Meaning dont state a password in wp-config file.
At least all the above is true for me…
Good Luck