Calculus is used by Isaac Newton to solve physic problems. Calculus helps to understand physical principles and it is used to study the degree of change. There are many rules, theorems and formulas which is not so easy to learn.
Memorize formulas and theorems immediately
When you are on the calculus class try to memorize as many formulas as you can. Do homework and always do exercise with calculus problems. Calculus is complex branch of mathematics and requires constant learning and practicing before you master all rules.
Practice daily/do self tests
If you want to understand calculus it requires every day practice. You can practice problems from your textbook and go to your professor to check it.
Do tests which you had at school under conditions which you had od class. Solve problems without books and notes. If class time is two hours, give your self the same time to solve your calculus tasks
There are many methods of self-testing, so you can combine problems from different exams and try to solve it. Work through questions from old tests and textbooks and this will help you to master the calculus.
Participate in study groups
If you work with your friends from class it can be both fun and useful. Be a part of the group that have intentions to master calculus. Do exams together and share your knowledge. Try to get answers by your own, at first, but when you are not able to solve some calculus problem, consult your classmate. If you have solved some problem by your own, show to your friend how he/she can do it.
Outline thoroughly for tests
When you are in school and professor talks about what you will have on the exam, you should outline that topics and formulas and theorems as well. This will help you to figure out how much knowledge you have and how much you have to learn. It will help you to discover your own weak points and you will be able to solve your problems on time.