Dropdown Box In Form

Hey all,

Here is my contact Form and Table:

Form "contacts":

Contact Name ___________
Phone _______________
Address _______________
Town/City ______________
State _____________
Post Code ____________

From the form the data is entered into Table "contacts":

id contact phone address town state pcode
__________________________________________________ _____
| | | | | | | |
| | | | | | | |
| | | | | | | |


And here is the Invoice Form:

Okay this is the bit where i get lost:

Form "invoice data"

Contact Name ___________
Phone _______________
Address _______________
Town/City ______________
State _____________
Post Code ____________
Job Date _______________
Due Date ____________
Description ____________
Amount _______________
GST _______________
Total Inc GST ___________
Invoice Number ________________

Notice that the first 6 fields (contact name, phone, address, town, etc) are the same as the fields in the previous table.

What i want is to be able to make the field "contact name" a drop down box, which when a contact is selected it changes all the other fields (phone, address, town, etc) to correspond with that contact.

I know that all that may have sounded a bit comlicated, but that is the easiest way for me to put it. Im sure this is really easy for you pro's.

Thanks heaps :)
ps. if unsure of anything i wrote plzzz ask :)