Many great nonprofits have been started by people whose main qualifications were energy and passion for a cause. However, success in the nonprofit sector is anything but guaranteed. Some important questions to ask yourself before you leap include:
Do some serious advance research, both by reading the many available print and online resources and talking to people in other groups. If possible, finding an organization similar to the one you have in mind and making a significant volunteer commitment to it can be the best way to learn, make personal contacts, and get ideas for how to proceed.
Also consider starting small. You can create a miniature, test project, just to see how things go. As a next step, before you apply for tax-exempt or 501(c)(3) status, you might ask an established nonprofit organization -- preferably one with a similar mission -- to become your "fiscal sponsor." This allows you to basically ride on its 501(c)(3) coattails, while it initially handles any money you receive. You can offer donors the right to tax deductions, and rely on the sponsoring organization to report and fulfill other requirements to your grantmakers.
To learn more about managing, building, funding and marketing your nonprofit, get Nolo's Nonprofit Bundle, a three book set filled with practical advice and step-by-step instructions to get your organization off the ground and maximize its chances of success.