The first step is to protect the idea. Have a competent business lawyer draw up an "Intelluctual Property, Non-Disclosure and Confidentiality Agreement." If you can't afford a lawyer, you may be able to find a form for that document online but that could be risky if there is anything about your idea that requires special attention.
Don't discuss the idea with anybody that hasn't signed your agreement. The agreement allows you to sue them if they steal the idea from you.
Without anything to show investors, it will be difficult to attract enough venture capital to pay the high cost of software development. I would put together a team of talented people who are willing to work on the project in their spare time for part ownership in the business. Form a partnership with the software developers and put the site together then go to work looking for venture capital depending on how much you would need to promote the site. Be careful about how much ownership you give away. The total should be less than 51 percent to insure you maintain control of the business. If you have to go over 51, do the difference with bonuses or incentives but not ownership.
If your business is about selling products, avoid getting into high fixed costs. There are manufacturers or wholesalers that are willing to drop-ship the product to avoid the cost of warehousing and inventory.
Remember what J. Paul Getty (who was the richest guy in the world at the time) said about how he made all of that money: Don't try to make all of the money for yourself. Develop long term relationships where everybody benefits from the success. Build a team.