I second the NOLS courses, and there are others available as well for wilderness first aid, wilderness first responder and EMT. The Red Cross offers good basic classes on CPR and first aid. Many local community colleges offer EMT courses.

As Hikermor says, assessing the situation and the casualty are critical. Learn how to look. How you look is very important in approaching any emergency situation. With experience it gets easier, more natural and more effective.