Many entrepreneurs immediately start developing their product or service -without taking the time to make sure that market need is real (and compelling), customers are willing to pay, and it's possible to deliver a solution - and make a profit at it.
Resist the temptation to dive into developing the product or service without documenting all of your business model assumptions and testing them first! You will save yourself much grief if you test the model first - and then develop the product or service.
An excellent way to start is to capture all your assumptions in a one page business model canvas.