Since you are in the US, I would suggest you start by reading the United States Code, specifically 35 U.S.C. 100-103. You can find the law and some explanations, as well as the upcoming changes, on the website BitLaw, or the website for The United States Patent and Trademark Office.
That said the answer will almost always be it depends. You can't patent laws of nature, abstract ideas, physical phenomena, or things that are not useful, or impossible, or even possibly offensive to public morality.
So whatever you are trying to patent must be: useful, novel, not obvious, and not anticipated by the prior art. Each of these criteria have been interpreted by US courts over the years to mean particular things, but knowing that those are the general criteria is a good starting point for you.