Filling a form

I'm using ASP.

I've got a form with a drop down list at the top from which you can select one option only. This works fine, but what I need to be able to do, is once I've selected an option is to read a database and fill in the rest of the fields on the same page.

Help how is this done as I'm failry new to the ASP side of things

see
http://www.asp101.com/samples/form_to_db.asp

Many other great samples here.

Unless you have all possible values loaded into client side arrays, you will need to pass the selected value back to the page as a querystring and populate the recordset only if the querystring is passed.
I would then say throw it into an array using the GetRows method of the recordset and populate the fields using the array.
You'd probably want to either set a flag indicating that a selection has been made or check array bounds so your code doesn't try to access the array when it is empty(On the initial page load)