Guide to Startup Forecasting

"Forecasting business revenue and expenses during the startup stage is really more art than science. Many entrepreneurs complain that building forecasts with any degree of accuracy takes a lot of time--time that could be spent selling rather than planning. But few investors will put money in your business if you're unable to provide a set of thoughtful forecasts. More important, proper financial forecasts will help you develop operational and staffing plans that will help make your business a success."

This article by Asheesh Advani provides some detail on how to go about building financial forecasts when you're just getting your business off the ground.