VBA data Capture in a Form

I have data in excel, Firstname, Surname, date of Birth and male - female. I
am using VBA with a form and Comboboxes. I can meke the dropdown to show the
Surname, I whant then for the Firstname box to only show the "firstnames"
only associated with a surname. Once the firstname is selected the date of
birth is populated into the "date of birth box". Can anyone point me in the
right direction?