If you have a new idea, but have yet to develop it, the best you can do is take basic precautions against theft of your paperwork and prototype. Even though you would like it to be true, your idea is probably not all that unique. Rest assured that others may have the same or very similar idea at the same time. It's how to implement an idea that is often difficult to envision. Most ideas are improvements on existing technologies or concepts.
Good ideas are solutions to known problems.
If a problem is widespread, it can be assumed that there are many who are trying to solve it. An NDA is a legal document that prevents outsiders from discussing the ideas they hear and the company business plans they see. The overwhelming majority of venture capitalists will not sign NDAs. If venture capitalists signed an NDA for every idea they heard, they would have to hire a team of lawyers to protect the business plans of the companies that they funded. As many proposals are similar to each other, venture capitalists would find their hands tied by loosely worded NDAs that they had signed earlier.