Schedule an appointment with an academic advisor prior to the upcoming semester. Tell the advisor what you plan to major in. The advisor will give you a list of classes that you need to take in the upcoming semester for your major. You can take more classes than your advisor advises you take, although you will need written permission from either the advisor or Dean of the school if you exceed the credit limit. For example, the credit limit at the University of Pittsburgh is 18; any student who wishes to take more must have written permission from the Dean.
Call the school or look at the academic calendar to find out when registration opens. Registration for the upcoming semester usually opens several months in advance before the semester actually begins, although each school's schedule varies.
Visit the registration office or register for your classes online. Most colleges offer online registration, although you can opt to register for classes at the registration office as well. You'll need your student ID either way.