What website can I search to find if my idea has already been patented?

On www.USPTO.gov, there are links for patent searches. You can search through both issued and published pending applications. Try the basic search and put in various keywords that would be used in a patent describing your invention. You can also try patents.google.com or even just a plain Google search. Depending on the technology, you could look in technical journals or databases like IE Explorer. Keep track of any vaguely relevant references you find as you should disclose these to the patent office when you file a patent application.

You can perform free patent searches at the United States Patent & Trademark Office's website, and just follow the patent links or via google.com/patents.

The US Patent Office and Google both offer free online databases of patents and published patent applications. A patent attorney can help you with a more in-depth prior art search and analysis of the prior art in view of the requirements for patentability.