Learn "blocking." This is crucial. In theater, "blocking" a play is the process of coordinating actor movement on the stage in order to tell the story of the piece being performed. In a film, blocking is the movement of people within the frame. Blocking is arguably the director's foremost obligation. He must create honest movements using the script as his guide, having actors move naturally as the play's action dictates it. This must be done while also avoiding blocking errors, such as having actors upstage themselves. Great directors have the ability to use blocking to create beautiful compositions on the stage and give the illusion of real life.
Analyze your scripts carefully. Before you ever work with actors or technicians as a director, you will need to immerse yourself completely in the script to be produced. This is to enable you to be authoritative with your directorial interpretation piece right from the start. Some playwrights and screenwriters include very detailed directions. Others include very little. Read the script and figure out what has already been spelled out for you and which gaps you will need to fill in yourself. Do not tackle the script as a whole, either. Break it down into "beats" and "units." A beat is a small chunk of the script that revolves around the same action or character objective. A unit is a collection of related beats.
Approach the theater holistically. Although a director's primary duties are to break down the script for staging purposes and block the play or film, he must have a basic knowledge of the other production areas as well. Since a theater director, for example, will have a great deal of influence over the set, costumes, and lighting, he must be able to speak competently with the production's set designer and builder, costume designer and lighting technicians. He must also be able to communicate with his actors in practicable acting terms. He need not be an expert in these fields (which in their own right take years to master), but he must understand them.