So.... You have an invention. First things first: All patents require innovation, but not all innovations can be patented. Patents apply only to a subset of all possible inventions. Democracy is an example of a terrific innovation that would not be patent-eligible. The patent-eligibility line lies somewhere between obviously non-eligible innovations like representative democracies and obviously eligible innovations like the artificial heart.
The first hurdle is whether your innovation is the kind of thing you can patent. Sometimes the answer is easy, but for a surprisingly large number of innovations, the answer is unknown. The language defining patent eligibility in the United States has been essentially unchanged for 220 years, yet the courts are still struggling to figure out where that line lies -- and the ongoing ambiguity is not one of precise line placement, but rather of where the line falls within a broad zone of ambiguity.(see Note 1 below and this article). The Supreme Court is taking another crack at figuring it out this year, and it remains to be seen whether we will have a better idea about where the line is drawn then (detail in Note 2 below) [UPDATE: Patent eligibility remains a murky area of law]. Until then, the best advice I can give you on subject matter eligibility is this: Stop trying to answer this question by searching online, because for a lot of inventions the answer is literally unknown. Hire a patent lawyer and just hope that the lawyer guesses right about what the law currently is and where it is going.
The next hurdles are far better defined. The invention must be (1) useful; (2) novel; and (3) not obvious. Again, a patent lawyer is your go-to person for an actual answer. If, at the time you filed for the patent (note that it is no longer at the time you invented the thing), somebody skilled in the art wouldn't have found your invention obvious, and nobody else has actually made the thing already, you're in good shape.
What if you can't afford a patent lawyer, or if you want to minimize your costs while you drum up investors (or think about the viability of the product, etc)? You can file a provisional patent application. A provisional patent application costs $130 (or in some cases $65) to file. It doesn't need to set out formal claims or follow most of the other requirements for formal patent applications.
Thanks to a provision in the America Invents Act, it is critical that you file at least a provisional application as soon as possible. Even if you invent first, if a later inventor beats you to the patent office, they will get the patent and you won't (with some complex and rare exceptions that your patent lawyer can explain).
A lot of the time people file provisionally without a lawyer, but doing so on your own does present some risk. In fact, while in some cases a provisional patent application makes sense, doing a provisional filing with or without a lawyer is not without risk (for example, if you use a provisional to postpone paying for a full utility application, you may find that your provisional application is missing some elements you later want to claim, and you might lose the priority date you think the provisional gives you). In some cases, particularly where there is a question about subject matter patent eligibility, you may be better off treating it as a trade secret. If you are ready to launch a product and want an issued patent ASAP, you might want to go with an expedited "Track 1" application. I know, to people outside of patent land none of that makes easy sense, but that alone is good reason to talk with a patent lawyer.
That said, I'm not a patent lawyer (I'm a licensed California lawyer, but my patent work revolves around developing and patenting my own inventions), and I can tell you that nearly all lawyers who work with patents lack the holistic picture inventors need. Basically, you're not looking for a patent just for fun (or if you are, that is some expensive fun). Rather, it needs to fit into your business plan. You need to be aware of costs, timing, and ultimately how your patent prosecution strategy impacts your ability to later monetize your patent (for example, by keeping competitors from ripping off your idea).
In the course of your patent's lifespan, you're likely to run into at least two legal requirements -- first, you need a lawyer to get your patent issued (yes, you can do it without a lawyer, just like you might be able to set your own broken arm without going to a doctor). That "patent prosecution" lawyer normally views the job as "get the patent issued". But a patent prosecutor doesn't know your business. Second, if you are lucky enough to get a patent that covers something valuable and unlucky enough that somebody else is ripping off your invention, you need a lawyer to enforce or license your patent (again, you can do it yourself, but it isn't easy to do it right).
One thing that your patent prosecution team won't do for you is think through the actual use of your invention in a way that leads to additional claims, patent elements, etc. Remember that your invention is not going to stand alone. Rather, the invention will take its place in an ecosystem where additional functions or small adjustments may be critical to success. Don't limit your patent filing to what you think is the most likely implementation and use of your invention. The future can be forecast, but is unwritten. You can easily be surprised to find that something you considered a minor piece of your invention turns out to be the primary source of value in your patent. You are already sharing your invention and creativity with the world -- you might as well spend some extra time making sure you don't leave the invention half done.
I should also point out that you have some enforcement choices. Please don't be a jerk about it. Some patent owners will threaten or sue individual end users of a product that is actually made by a bigger company. Technically, those individuals are infringing, but you should think long and hard about whether it is right to go after individuals and small family businesses before going after the entity that supplied them with the infringing product. Sometimes it makes sense, but it should be done only after giving a lot of thought to whether it is the right way to go about it.
I know I said it a bunch of times already, but to reiterate: See a patent lawyer. Seriously. A seemingly tiny error in drafting can mean the difference between validity and invalidity of a patent.
Note 1: The U.S. Constitution authorized patents ("Congress shall have the power... To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries...", Art. 1, Section 8, Clause 8). The Patent Act of 1793 described the kinds of innovations eligible for patent protection very broadly: "any new and useful art, machine, manufacture or composition of matter, or any new and useful improvement on any art, machine, manufacture or composition of matter". The language in effect more than 220 years later is nearly identical: "any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof, may obtain a patent therefor, subject to the conditions and requirements of this title".
Note 2: Bear in mind that the Supreme Court normally determines what the language in the statute has always meant -- meaning that filing for a patent prior to the Court's decision is unlikely to make any difference. If the Court decides your innovation is not patent-eligible, the decision will apply whether you already hold an issued patent, whether you were previously on file with an application, or whether you have yet to file an application. Congressional changes to patent law (and yes, they're also likely to make some big changes to other parts of patent law) are different, and for those changes filing dates frequently matter.