Heavy weather sailing
There are all kinds of sailing lessons. Some include only classroom instruction and perhaps a few hours of sailing on a Sunfish. In other sailing courses, a considerable amount of time will be spent sailing a sloop-rigged boat. (i.e., a boat with two sails; a mainsail and a jib.) You need sailing lessons on the general type of sailboat that you plan to sail.
If you have had any kind of sailing lessons, you have undoubtedly studied the fundamental principles of sail trim, but reading about them in a book and actually putting them to use on a boat are quite different things. It would be very helpful to you if you will volunteer to crew for an experienced sailor, and ask him/her to explain how to trim sails, while you are underway. Principles of sail trim make more sense when you can see them in actual use. Hang in there! It was all a mystery to us when we were first learning to sail.