Write or call each department that interests you. Ask for whatever information you need and for application materials. Try to have all your letters sent out by Mid-November. Take the Graduate Record Examination as early as possible in the Fall semester. If you are rusty in math, do one of the GRE study courses.
Acceptable scores vary from school to school.
Most use them as only one indicator of your probable success. During Christmas break, fill out the applications and send them in. Most schools decide early in the Spring Semester about admissions. The longer you wait, the greater your risk of not getting in where you want to or not getting in at all. Top notch students sometime fail to get in schools for which they are more than qualified just because of this approach to admission.