I would also recommend Drupal if you want to build your own website from scratch with unique business. Advantage with Drupal is that is can be completely customisable and experienced Drupal developer can build MVP of products that compare to Squarespace, AirBnb, LinkedIn etc.

Many times I have been asked to sign a NDA. I never sign that kind of agreements, not because I want to stole ideas, but because agreements were too general and I could potentially lock my self out of doing related projects. That said, don't be that person who throws NDAs in front of people. Idea is the 1% of the whole business. If it ends up a successful money making product you will get the competition soon anyway. Whatever you are doing, you have to be a couple of months ahead of them. The other point is that people are too lazy or too busy to start a new venture anyway. If you seek advices from entrepreneurs chances are they are already busy building their startups. Also, being first does not make you better by default. If you take a look at examples of today successful companies you would be surprised how their current business model differs form their initial idea. Best of luck!