From Wikipedia:
In a fully upright stance, the boxer stands with the legs shoulder-width apart with the rear foot a half-step behind the lead foot. Right-handed or orthodox boxers lead with the left foot and fist. Both feet are pointed slightly inward, and the right heel is off the ground. The lead (left) fist is held vertically about six inches in front of the face at eye level. The rear (right) fist is held beside the chin and the elbow tucked against the ribcage to protect the body. The chin is tucked into the chest to avoid punches to the jaw which commonly cause knock-outs. Some boxers fight from a crouch, leaning forward and keeping their feet closer together.