Hi. You should consult an IP attorney for your agreement. Typically, the creator owns the copyright. But, you can hire a creator and have a contract that stipulates you, as the employer, owns the copyright. Also, you should use a reputable company, preferably a U.S. based company. If you use someone from a remote region of the world, and they breach, you will have a difficult time enforcing the agreement.
To be clear, you cannot protect your "idea." The general concept of your online business is not protectable. What is protectable is the trademark (brand, domain, product/business name), copyrightable material (content, images, etc) and perhaps a patent (pertaining to some novel, non-obvious invention) although is not likely relevant.
You can employ an NDA/Non-Use agreement but these need to be drafted and balanced carefully if they are to be enforceable. And even then they are not going to be useful regards to protecting a general business concept.
As my colleague aptly notes, using a provider overseas will for example make any enforcement of your agreement next to impossible.
You may want to discuss this and other legalities over with a lawyer in private. Most of us here, including myself, offer a free phone consult.
The law firm of Natoli-Lapin, LLC (Home of Lantern Legal Services) offers our flat-rate legal services in the areas of business law and intellectual property to entrepreneurs, small-to-medium size businesses, independent inventors and artists across the nation and abroad. Feel free to call for a free phone consultation; your inquiries are always welcome: CONTACT: 866-871-8655 Support@LanternLegal.com DISCLAIMER: this is not intended to be specific legal advice and should not be relied upon as such. No attorney-client relationship is formed on the basis of this posting.
So, what you need is an NDA and a non-compete, I think. If you were just publishing content that you didn't want this person to republish or share with other parties, an NDA would be enough on its own; unfortunately, what you want to protect is the "idea" of the site - that isn't protected by copyright, so there's nothing to prevent this web designer from capitalizing on that idea in some way unless a valid non-compete is in place prior to the beginning of work.
I don't know that it's really worth going to that extreme to protect the idea of a site; once yours goes up, you'll have competition very quickly if it doesn't exist already. I think the best you could reasonably expect is to protect your copyrighted material through the combination of copyright and a non-disclosure agreement and work with a reputable professional within the United States so that you reduce the risk of something going wrong and have adequate remedies should that professional breach your contract or NDA.
You should definitely see a lawyer about the various options you have to protect yourself and to determine what your best course of action would be.
No information you obtain from this answer is legal advice, nor is it intended to be. You should consult an attorney for individualized advice regarding your situation. No attorney-client relationship is formed by my responding to your question.