NDAs are (mostly) dumb
You've got a startup idea you've been thinking about for a while and it just smells like a winner - this could be the next Airbnb or Stripe. Awesome!
Leaving your job to start a company sounds really scary, so before you really start working on the idea you decide to get some advice. Awesome!
After a bit of research you reach out to a few experienced founders and VCs who have proven they can build companies. One of the founders responds to your email and says she's keen to help! Awesome!
She asks that you send her a quick overview of the idea, market size and anything else that would help her understand your idea better. You reply and say you're happy to share your idea if she signed the NDA you've attached. Not awesome...
Why NDAs for your startup idea are stupid
Let's consider this thought experiment for a minute: You write a detailed 20-page description of your idea, including diagrams and screenshots, a roadmap of features for the first year and your exact marketing plans to launch. Once you've perfected the business plan you print 1000 copies of the report and start distributing them at tech meetups and conferences over the next few month.
Here's what I think will happen: Absolutely nothing.
Nobody will pay attention. Nobody will copy you. Nobody will mention you in the press. Nobody will give any shits, at all. Welcome to entrepreneurship :)
The biggest risk to your startup is not competition
The reality is that your idea probably sucks balls. You think it's great but you haven't thought through all the problems you're going to face as you build the company. How will you acquire customers? Who will you hire that can sell to those customers? What marketing spend can you afford at your sales price? How do you roadmap software? Which VCs are interested in your space? How will you get an intro? Do customers even really want the product? How big is the market? etc...
Building companies is really hard - make it easy for other people to help you. Don't make them sign an NDA.