MySQL Port Number
Here is a quick tutorial on the MYSQL Port Number.
If you are performing a default MySQL Installation, the database port defaults to 3306. So incase you are trying to provide an input to software like Wordpress or Vbulletin to install the database, you can leave it blank unless your isp has given you a special number. Leaving the mysql port number to default will automatically work against 3306.
To find the MySQL Port Number on a server where the port number has been changed for specific setup or security reasons, you can refer to the /etc/mysql/my.cnf file. In this file you will find the port number specified.
For users that are using a mysql client on unix / linux without any guy, you should use a --port=xxxx to change the connection to work against the new port number.
Hope this helps