Adjusting Data Connections

I am currently trying to link a form to an online MySQL database.
The table that I am linking has a row for every person's data.

I would like to have a dialog box that requests the username of the person that the user wants to view and then use the value from the dialog box to adjust the data connection to use the MySQL statement:

Select * from personalData where username = 'xxxxxxxx'

I'm not sure how to start.
Please help