Ideas that you've dreamed up in your head but have yet to place into tangible form can't be patented. In order for an idea to be patented, it must be more than theory. It can't just be the starting point of an investigation; it should be the culmination of your research. You should know the exact steps needed to make your idea a reality. But even more than that, someone of equal skill and knowledge should be able to look at your patent application and use your specifications to create the exact same thing in a workable form.
In order for an idea to be patentable, the invention represented by the idea must be useful. In patent terms, this is referred to as having utility. One way to show that something is useful is by creating a prototype. A prototype is essentially a model of your invention that demonstrates your design and how it works. It's good to make a prototype before you submit a patent application because you'll likely discover flaws that need fixing or come up with improvements for your design. If your invention is too expensive to create a working prototype, you can create a virtual prototype with computer animation software.
Just because you have a working prototype doesn't mean it's patentable. You'll still need to meet a few additional requirements. Your invention should be novel, meaning it's new and hasn't been created before. It will be deemed new if it provides a substantial, significant improvement on an already existing process or invention. There shouldn't be any prior art, meaning that nothing should have been published about the invention before you file for a patent. It also should be nonobvious, which means that the invention shouldn't be obvious to someone in your field. Rather, your work should be a surprising creation or result in a surprising outcome.
Proof of Ownership
When your invention is still in the idea stage, you can't patent it. However, there are other steps you can take immediately to ensure you retain ownership once you're ready to patent it. In order to own a patent, you'll have to prove that you came up with the idea first. Take detailed notes from the moment you have the idea, documenting every step in the creation process. Write down everything about the invention, including how you intend to make and sell it. Keep all your ideas in an inventor's journal that's signed and dated by you and a witness. Make sure the pages are numbered consecutively and no pages can be removed or inserted. This ensures you'll have a clear line of evidence showing that you are the author of the idea and your timeline of creation.