It does not have to be long or overly complicated. It simply has to have the elements required to put your goals into action. Developing a SWOT analysis (strengths, weaknesses, opportunities, threats) will help you to identify problems before they start. Craft your own or hire a business writer to create a dynamic plan that will guide your operations. An effective plan is one of the most important elements over overall business forecasting.
When you, and your partners if any, have all the core skills and industry knowledge you need to start right away without seeking experts, napkin notes may be enough to get going. Let's say you are already an expert in technology and social media. Then you, and your team, probably don't need a detailed plan to start developing a new app. You will draw on your knowledge and experience, and you understand that you might need to go back and do some more detailed and formal planning later. Certainly when you reach the point where you are looking for investors or lenders, you will move beyond those first casual notes. Until then, drawing upon your expertise can allow you to quickly jump into the market and perhaps gain a competitive edge by using a minimalist plan.