Use of mainsail

That's great that you are going to take lessons, you will enjoy it and will learn faster the correct way to sail. get the book that the school uses and start reading. most cruising boats are monohulls that do not plane and are limited in speed by the hull speed of the boat (Max Speed it will go). so when the wind is blowing hard you can be over powered by to much sail which can make it harder to control the boat and you will not go any faster then hull speed. it does depend on the type of boat and the type of rig, some have large mains compared to the jib and some the other way around. most boats will sail better with both sails but every boat is slightly different.