Study books and guides that explain the calculus skills you don't understand clearly. The materials you choose should serve as supplements to your course textbooks. Contact a calculus professor on campus or check your college's mathematics department website for a list of appropriate books and study materials.
Take additional college courses in order to master the fundamental math skills used in calculus. You should consider doing this even if you have already taken these courses in high school. College-level algebra and trigonometry courses -- not to mention so-called pre-calculus classes -- can provide you with the skills necessary to succeed in calculus.
Learn from a tutor. Almost every college campus offers tutors in a variety of subjects, including calculus. Sometimes the university hires tutors and other times you must hire one yourself. Studying with a tutor allows you to focus on the calculus skills that you may be having trouble mastering.
Use the online resources found on Calculus.org to identify your weaknesses and improve your skills. Math professors from over five different universities and math centers designed this site to benefit both math students and instructors.
Join a study group. Students in calculus classes commonly create these groups to facilitate the learning process. If you aren't currently in a calculus class but still want to work on your skills, visit a mathematics professor on campus and ask him or her to point you towards a specific group.