Web Service or not? How do I accomplish this?

[Tom]

I’ve been reading and reading and I need help on the best way to approach
this. I need to develop web site and/or web service that accepts connections
from a few “allowed†sites. After It is determined that the incoming site is
one of the “allowed†sites I need to read the information sent in (I would
like this to be XML data that I can compare against a XSD file). I the need
to write that information to a database and send it off to another service
(similar to PayPal), I then need to read the response from the “PayPal likeâ€
site and send that response to the original calling page. I’m completely
lost as to where to start.
Thanks,
Tom