Ask a school adviser or counselor what topics are taught in the school's pre-calculus class. Your school may also have information available on its website. Some schools use different names for their pre-calculus class such as Algebra 3, Introduction to Analysis. Your school may cover the topic as part of their algebra and trigonometry classes. Some of the pre-calculus topics are real numbers, complex numbers, inequalities, quadratics, rational functions, polynomial functions, trigonometric identities, exponential functions, logarithms, vectors and matrices. Make sure you haven't already covered the pre-calculus topics in a previous math class.
Investigate the requirements to sign up for pre-calculus by asking an adviser, counselor or looking on your school's website. Typically, you need to have already taken algebra and trigonometry. Some schools only require that you have passed the previous classes while others require a minimum grade, such as a C. If you are entering college you may have to take a math test first to see what math classes you are allowed to take.
Sign up for a pre-calculus class through your adviser or the school's online scheduling tool, if available. Sign up as early as possible so that you get a spot in a class and have access to choose the class with the best teacher, location and time. Join a waiting list to get in the class if necessary and check it regularly, as many students drop out around the first week. It's sometimes possible to get in a full class by nicely asking the teacher for permission.