That's a tough question. Usually in order to get a patent you do not have to build it but you should be able to show technical diagrams and figures showing how you would build it. If you are not a technical person, you may have to find an engineer or someone to team up with you on the project.
One option is to file a quick provisional patent first listing as best you can how you would make the device (this would only cost $65 for most people and would try to protect just the general idea that you have), then later if you were able to find some technical help you can follow-up with a more technical provisional patent application. The downside to this strategy is you would have to pay two different $65 fees at the USPTO (one for each provisional patent app that you file).
In any event, it would probably be best to at least get some sort of patent on file before you pitch your idea to a company. Even if its not a perfect patent application it may be good enough to make them think twice about trying to steal your idea.
Kind regards,
Brad