How should I market on the internet to develop a datbase of people in
an area to register for a neighbourhood watch

Snyone --

Obviously your website would want to include Meta tags and keywords
(in HTML headers, text and even image tags) to cover the neighborhood,
it's streets/parks/businesses.  But since the neighborhood and the
Block Watch itself is defined by the people in it, why not consider a
wide program of notifying people of the URL on posters, flyers,
business cards and other print material that can be distributed
locally?

Best regards,

Omnivorous-GA