Take college courses to gain certification as a teacher or coach. Depending on the state, certification requirements will be different, and not all licenses are transferrable to other states. Generally speaking, the coursework consists of coaching theory, educational theory and emergency preparedness. Even before earning your degree, these courses will establish you as a professional in the field---listing these courses on your resume can help you get temporary coaching jobs at smaller schools, while you work on your degree.
Establish yourself as a professional coach. Join professional organizations such as the National High School Coaches Association (NHSCA). You do not need to be a fully licensed coach to join the NHSCA. Establishing professional affiliations early on will make it easier for you to find and land a job---many organizations, including the NHSCA, help their members find jobs. In addition to national organizations, get plugged into the local coaching population with state organizations such as the Minnesota State High School Coaching Association or the Texas High School Coaching Association.
Contact private schools about upcoming coaching vacancies. Many public schools require coaches to be licensed teachers, and they typically offer coaching jobs to educators already working in the building. But small private high schools may have public openings for coaches. Opportunities for advancement may be limited, but the jobs provide good experience while you work toward a degree or wait for a coaching vacancy at a larger, public school.
Volunteer while you wait for a coaching job. Even if you can't land a paid high school job right away, volunteer experience at community recreation centers will carry weight on your coaching resume. Any experience is good experience, whether it's coaching, assistant coaching, refereeing or scorekeeping. Volunteer opportunities also get your foot in the door, making you first in line to hear about new coaching opportunities.
Consult your state's Department of Public Instruction website for coaching vacancies. Such websites contain regularly updated databases of teaching and coaching jobs in the area. For example, Wisconsin's WECAN database lets certified coaches search for jobs. Many states have public databases open to job seekers, making it easy for you locate lower-level coaching jobs while still in school; for example, North Carolina's DPI has an extensive library of public job listings.