You do not need to build a prototype of your invention before filing a patent application for your invention with the US Patent Office. You are only required to explain your invention in sufficient detail so as to enable one having ordinary skill in the art to practice your invention.
That means that you have to at least be able to describe your invention sufficiently that someone having experience in that area understands your invention. But you do not need to actually build your invention before filing a patent application.
For example, if your invention is for software (a mobile phone application or “app”), you do not need to be a computer programmer to apply for a patent application for your software invention. You only need to be able to describe to a computer programmer what you want the software to do. But you do not need to know how to write the program code yourself. You do not even need to hire a programmer to write the software for you. Just be able to explain it to a computer programmer who could then write the software code for you based on your explanation without having to figure out how to make it work. If you can do this, you can apply for a patent application for your software invention.
That means you can go invent a mobile phone app, or even a mobile phone itself, an electric car – really just about anything – even if you have no way of making it yourself. You can then file a patent application for your invention even though you have never built, and may never build your invention.
So what are you waiting for? Go invent something!