If any part of you is in the neutral zone, you are offside. The center is the only person allowed to be in the neutral zone, which is an imaginary strip the width of the ball and extending from sideline to sideline.
The exact definition, from the NFL rule book, is:
SECTION 20 - OFFSIDE
A player is Offside when any part of his body or his person is in the Neutral Zone, or is beyond his free kick line, or fair catch kick line when the ball is put in play, unless he is a holder of a placekick for a free kick (6-1-3-b-1) or fair catch kick (11-4-3), or a kicker (6-1-3-b-2). The snapper is offside if any part of his body is beyond the neutral zone. The kicker is not offside unless his kicking foot is beyond his Restraining Line when the ball is kicked.