Power boats are very different to manual boats, and if you have sailed a non-powered boat, the skills will not necessarily be interchangeable. It you would like to learn how to drive a power boat, you will need to get a specialist qualification, often thought of as a driving license for power boats. You may be able to train on the job in some industries, however if you are just learning for fun, you will need to take a course.
One of the best courses to take is run by the Royal Yachting Association (YWA), which has bases in various places. You can go along to one of these clubs to take the certificates you need. You will normally begin with an introductory course, which will teach you the basics across a couple of days. If you then want to make this into a formal qualification, you will need to book a power boat test at the end of it.