You sign up for courses during your orientation. You meet with an advisor who helps you select and actually sign up for courses. You can start looking through the course catalog now if you want. If you have already picked a major, be sure to look at the curriculum specific to that major (which is usually found on the department's website). Also, look at the graduation requirements to see if you can start fulfilling any of those now. Keep in mind any course requirements that may be associated with a specific group (like Honors College).