build it
get users
get funding
(why this order? its easier to get funding if you have users. also get funding! its hard and expensive to run a company)
expanding on the build it:
get a template you like from themeforrest.net (really $5-20 for a template, is amazing, people spend $80,000 for a design and website or app... )...just get one that matches your site function close enough, its only $10 so you can change it later
start building your app and site or product
build a MVP - Minimum viable product, basically a stripped down simple version (not every whiz bang feature you could add). The goal is to build this in a few weeks and get it out there so you can see if its going to take off or not. (theres tons of articles on MVP).
edit: the other posters idea about testing the idea is very valid also. there are other ways to test it too, like putting some sample ads on craigslist or ebay and seeing if you get any calls. or putting a landing page (emails)/order page and seeing if you get any orders (then cancel them and apologize) to test the validity of the idea.
Go meet in person and talk to 5 to 10 people in your target market. E.g. pay them a cup of coffee, or set up a 15-minute meeting at their business place. Ask them what their major pain points are, and validate that your solution would be valuable to them. You'll know that if they (at least informally) agree to pay you $x/month for such solution.
You'll either learn a lot about what's really important to solve, or find out early that there's not a market for it.
[1] http://www.inc.com/steve-blank/key-to-success-getting-out-of...
These might help: