Use your owner's manual and learn all you can about it. Some are simple and will only take a short amount of time. Others are very complex and will take much longer. Entire courses are offered for some of the scientific calculators. You may want to check to see if one is offered in your area.
Calculator skills are vital for many math classes. Even if you have teacher who won't let you use one in class, it can still help you check your work at home.