How to search the database?

Hello.. I made my Login and registration pages and everything is okay.. but what I want now is to know how to make this : A PHP page for the administrator.. that has a TextBox (UserName).. he will enter a user name in it.. after that he will press the button (Activate).. Now by pressing that button the PHP page will search the database (A specified table) for that username, if it is exist then search his row for a field named (Activation) and then update its value into "Activated".. but if this user is not exist in that table.. a message will apear for the administrator.. Waiting your replies guys ;-) Thanks in advance... Cheers..