Find out what the restrictions are in your state. The laws on riding motorized bicycles vary greatly from state to state.
Decide on an electric or gas motor. Generally speaking, quality gas motors are more expensive and require more maintenance but will travel faster and farther while electric motors are less expensive and require less maintenance and travel shorter distances at lower speeds.
You also need to decide on whether to purchase a kit to add a motor to an existing bicycle or just buy a fully built motorized bicycle.
Now it is time to find the product. If you live in or close to a big city, you may find a dealer with a showroom. Check with local bicycle and scooter dealers to see of they carry what you are looking for.
If you do not find anything locally, do not despair. The Internet is loaded with manufacturers of both kits and pre-built motorized bicycles.
If you are not mechanically inclined, seek a second opinion on your chosen bicycle or kit before purchasing. The right engine can make or break your experience with your new motorized bicycle.
If you are satisfied with your selection, then it is time to buy and begin enjoying your motorized bicycle.