Linking to a Header File

Hi

I have a header .html file. I am using an online service which points
to this header file and as a result reproduces the header file at the
top of each page, making the 3rd party web pages look like my own.

What is the best way in dreamweaver to get my own pages which I have
created in dreamweaver to aslo point to and use the same header file?
I am trying to get to the point that I only need to update the 1
header file and all pages, wheteher created by me or 3rd party reflect
changes to the header file?

Any help would be greatly appreciated.

Callum