Can I protect the idea of a new web service (site), if I want to sell it?

You can protect a detailed plan for a new and non obvious web service by obtaining a patent for it. The patent can then be licensed or sold. In the absence of a patent a nondisclosure agreement can sometimes help if you wish to negotiate with someone that is interested in licensing a patent for your web service, if it becomes patented. It may be difficult to commercialize your web service invention without patent protection.

You should have a conversation with an intellectual property attorney to discuss the details. If you think the idea is new, it is probably not safe enough to provide the details here to get the answers you need.

Yes a non disclosure can protect the embodiment of your idea (the invention) However if you are that advanced i would suggest that you file a utility patent on it or if money is tight, then just file a provisional patent on it with as much info as you can put together.