Q: What should be an entrepreneur's first step when creating a business, product, or service?
A: Write a press release (PR) and frequently asked questions (FAQ) document.
Think: Begin with the end in mind.
The PR and FAQ are notional and for internal use only. The PR focuses on your product's benefits and the FAQ answers specific questions regarding features and details. Later, when you're ready to ship your product, you'll publish the actual PR and FAQ for public consumption. In the mean time, the notional PR and FAQ are used to socialize your vision with the team. Sure, you'll tweak the document, slightly, while you're working on your baby, but by using this as a starting point... as your vision document... gets everyone on the same page and it keeps the founders and team from getting distracted.
The PR should be a simple one or two page document describing the benefits of your product and the FAQ can be a few pages. If, later, you find development straying from that notional PR, then you'll either need to update the PR or ignore the distractions.