filling the textbox,dropdownlist from the database??

hello everyone..
 i have a question..may be you can help.
i have a web form with server controls like textbox, dropdownlist, radio button, label etc.its a user registration form..

now what i want here is..when ever i choose a option from the dropdownlist. i want all the controls like textbox, dropdownlist etc to be
automatically filled up by the data related to that option in the dropdownlist..i want this data to be fetched from the data that is stored in the sql database.
how is it done..
 
anybody??
 
thanks.. 
 
 
...