Position your actor on stage by directing him to stage left or stage right. Left or right is from the actor's point of view, looking out into the audience. This is the reverse of what the audience, or the "house," sees.
If the director is watching the play from the seats, and he/she wants the actor to move more to the right, he/she will tell the actor to move more "stage left."
To put it simply, stage left equals house right, and stage right equals house left.
This is so all the actors on stage have a common frame of reference. Depending on the scene, there could be many actors facing many different directions, and each one would have a different version of left and right. By using instructions like "stage right," a director can tell everyone to move a step toward stage right, and everyone will know where to go.
Move your actor forward or backward by directing him/her upstage or downstage. If you want your actor to come toward the audience, tell him/her to come downstage. If you want your actor to away from the audience, tell him/her to go upstage.
These terms come from the Shakespearean days of theater. The stage had a severe tilt, also known as a "rake." This allowed for better visibility for the audience. The part of the stage closer to the audience was literally lower than the part farther away. Upstage and downstage are very easy to understand in this context.
If actors are supposed to leave the stage, they are told to exit. To come on, they are told to enter.
Many theaters have legs on each side of the stage. Legs are curtains that hang parallel to the front of the stage and are used to mask the backstage area and wings from the audience's view. A director may ask an actor to exit in 1, 2, or 3. These refer to the openings created by the legs on each side of the stage. The most downstage entrance is 1, and the numbers increase as you go upstage. If a director tells an actor to exit stage left in 1, he means for the actor to exit as far downstage as he can on stage left.
Actors don't just move from stage left to stage right--they "cross." If a director wants you to move from one side of the stage to the other, she/he will tell you to cross to stage right.