I have a great water filter, do I need to get a patent to start building and selling it? What is the check list that I need?

A patent is not a license by the government to make a product. You can hypothetically patent things that are illegal to manufacture or use. If you have a new invention, that you invented and is not obvious to the average person in the water filtration business, then it is possible you might be able to get a patent. A patent allows you to exclude others from making / selling / using the invention described in your patent, depending on how it is drafted. First take a look at the USPTO site to make sure your invention is not already out there. Next you should contact a "patent attorney" to review your invention. Patent attorneys are registered to prosecute patents with the USPTO.

A patent is probably one step of what you'll need. You will probably also need a business entity, such as a corporation or LLC. But I would think that if all you have is the engineering, there are a lot of things between a good idea and a successful manufacturing or technology-based business beyond legal advice. You might want to talk with the local SBDC or talk to J.J. at the Eugene Chamber. Or if you're working with technology developed out of the University of Oregon, you'll probably need to work with Tech Transfer. You'll probably need a lawyer at some point along your path, but I wouldn't start there.

It is essential that you file the patent immediately. The patent rules require that the patent application be filed soon after the discovery of the invention ( I believe the usual rule is one year). You should immediately contact an attorney to start the process.