When you set up your mySql database you will have a hostname (usually 'localhost' will be correct), the database name, the database username and the password. These should be entered into your localhost.php
Instuctions are here:
Upload this to your remote server when you have made the changes.
You can post the contents of your localhost.php (in your 'Connections' folder) when you've done, if you want someone to check it over. (Put xxxxxxxxxx instead of your password, as this is a public forum!)