You don't mention if you are interested in a part time or full time ski instructor position. Either way, I believe that your best bet is to join a local area's ski instructor staff and go from there. Most resorts in this immediate area do not require PSIA (Professional Ski Instructors of America) certification as a condition of hiring. In fact, many local area instructors do not get certification until after they start teaching at a local resort. I know Ski Liberty regularly hires new instructors every season. You may want to check their website for information on signing up.
Certification in the U.S. is available through the Professional Ski Instructors of America - PSIA. Their web site is www.psia.org and contains information on certification and standards.
There are basically three levels of certification, Level I, Level II and Level III. Each level of certification requires that you pass on-snow skiing and teaching examinations as well as written examinations.
In order to take each examination you must have a form signed by a ski school director indicating that you have the minimum qualifications to take the exam. For a level I certification, this basically requires a small minimum number of hours of teaching. The level I certification is easy to obtain for an accomplished skier, after that things become considerably more difficult. Most Level III certified instructors fail the exams the first several times before they get their skiing and teaching to the level the examiners require.
If you are interested in becoming an instructor, it is fairly easy to find ski schools interested in part-time instructors.
Assuming that you are local to D.C., Ski Liberty, Whitetail, Roundtop and Massanutten are the closest mountains to D.C. and I know that Whitetail and Liberty both have a real need for new instructors.
I teach at Liberty. It is the closest mountain to D.C.
Most mountains have an instructor training course that includes both classroom instruction and at least one day of on-snow.
You will have to pay around $100 to take the course. If your skiing is up to par, which it probably is, you will be offered a position.
Most new instructors are able to take the Level I exam near the end of their first season of teaching.