How to approach a web and app designer developer? [closed]

I have an idea for an app and a website that is needed to run the app. I need to make a cost report to find investors and to find the feasibility of implementing it. I would like to call web developers and app developers but I do not have any knowledge (I know about it roughly, but not the important stuff) about it. I also do not know the latest technical terms. I would like to get quotes regarding the cost, but I do not want to leak my ideas. Please give me some advice on what to ask, what to study to not be conned, how much to give away, how am I protected from theft, and other important stuff. Thank you.

I have made a static HTML website, but that is the max knowledge that I have. I know you can make a website using Wix, but that too is mostly static. I want to make a dynamic website where I am expecting at least 400-500 users working on it and the app to have a lot more users viewing it.

P.S. this question was asked 9 years ago on this website, but the links are now obsolete.