It's best to start with an early validation website, showcasing an MVP or at least testing interest and pricing with sketches or other mockups.

You could use one of the various landing page websites out there like leadpages, kickofflabs, etc. and have an email sign up form. From there you can email updates to your list of early adopters.

Buffer's MVP story is an easy to follow example: https://buffer.com/resources/idea-to-paying-customers-in-7-w...