After selecting a floor plan, contact your local building department to check any zoning, or minimum square footage, requirements. Ask your building officials what the requirements are for filing for a permit. This is a good time to start interviewing contractors. Some building departments will require a local engineers stamp on the drawings, a local contractor should be able to help you determine permit and code requirements in your area and direct you to licensed professions if necessary.
To find a contractor, contact your local Chamber of Commerce or the local branch of the National Association of Home Builders (click to visit their website)