Get licensed by a private or on-line boating course
Completion of a boating safety course, sometimes called a boating exam, is a requirement in many states to operate a motorized vessel. A test will be given at the end of the course. If you pass it, you'll get a certificate that many people refer to as a "boating license", although technically it's not a driver's license like the one you get for driving a car. You can operate a boat with the certificate.
You may need to show this certificate to a law enforcement officer, especially the U.S. Coast Guard, or government agency to register a boat. If the course is approved by NASBLA – approved by the National Association of State Boating Law Administrators and recognized by the Coast Guard – it will be good for life and qualify you to operate a motorized boat elsewhere, even if you move to another state.
If you don't need to take a boat safety course, it is still a good idea to do so, especially if you have recently moved to a new area and are on unfamiliar or new waterways (saltwater bays and inlets, for example instead of freshwater lakes) or after a long absence start boating again. Additionally, it's something you can do as a family, perhaps as a refresher for adults and as a primer for teens. The course will bring you up to date on all aspects of boat safety, navigation, boat operation log, local laws and related topics.
Online versus in-person
You can take an online boating course or take the course in person at a local venue. But the first thing you need to do is find out what it takes to operate a motor boat in the country where you live. In some states, only young operators are required to take the course. Next, see what your state accepts as a valid course, generally approved by NASBLA. Even if it is not required, it is a good idea to take such a course for the reasons stated above. Your government agency can refer you to approved online and in-person courses.
An online boating course is particularly well suited for those who have limited free time, live far away from the nearest personal course, need to get certified immediately, and cannot wait for a personal course at a later date, and who cannot be carried out on the date or dates when it is available on site. Since you can take an online boating course in your free time, you can also do so gradually, at night or whenever you have free time. Just make sure you do this at a time and place where you have a good internet connection as the course will take several hours to complete before the exam.
In-person boat safety courses are offered several times a year in many locations by local US Coast Guard aid chapters as well as some major outdoor supply retailers. They follow two regimes: a roughly six-hour course held in one session, usually on a weekend day, or a two-session course held on (usually) consecutive evenings, with each session lasting about three to four hours. Both are followed by an exam. I recommend taking a course that can be done in one session, especially if you have to travel a distance to get to where the course is offered.
In the case of a personal course, the administrator can adjust part of the content to take into account local or regional navigation situations and specifics, which is not the case with a more general online course. You can also learn something valuable from other participants in a personal course.
With your certificate in hand and a fishing license, when you're ready to fish, you can get on the boat and start the engine.