Even a good business idea might not be financially workable. To learn how your idea will fare, you should prepare what's called a "break-even analysis." In a break-even analysis, you project income and expense estimates for a year to determine whether, in theory at least, your business will make enough sales revenue to pay its expenses.
A break-even forecast includes the following:
If you find your break-even revenue represents an amount of work your business can handle -- that is, if you can easily bring in more than the amount of sales revenue you'll need to meet your expenses -- then your business stands a good chance of making money.
For more information on preparing a break-even analysis, see Will My Business Make Money?
To learn more about how to choose a business that will succeed, read Running a Side Business: How to Create a Second Income, by Richard Stim and Lisa Guerin (Nolo).