retrieve single row data in mysql using php
===

I created a website that has multiple logins and unique informations to it.. i want to retrieve the data from one user. example my username is qwert and my password is 1234, I want to retrieve his unique information to the database. I used the sample code in w3schools(http://www.w3schools.com/php/php_mysql_select.asp) and it selects all the data but all I want to do is to retrieve the data from the user which is only logged in. can anyone help me about this? any help will be much appreciated. by the way sorry for my bad english =D