Exercise regularly to keep yourself healthy and in shape and avoid smoking or inhaling second-hand smoke to keep your voice in good condition. Warm up before any rehearsal, performance, or solo practice. Your warm-up should include basic stretches of your arms, legs, and core, and some sort of cardiovascular activity such as a quick jog or jumping jacks. Warm up your voice before your rehearsal; you can use tongue twisters or recite your lines to yourself.
Open your body to the audience whenever you're on stage (unless specifically told otherwise by the director). While it is typical to face a person directly when speaking to them, on stage we open (or turn) our bodies to the audience so we don't cut them off from the scene. It is also important to "find your light" while performing; this means staying in your lights so the audience can see you clearly. A good tip to stay in the light is to pay attention to the heat of the stage lights on your face. If you can't feel it, you've likely stepped into the dark.
Learn some theatrical lingo before embarking on your first theater audition or rehearsal. "Blocking" is a term for the movement of the actors on the stage. Your director will give you blocking toward the beginning of a rehearsal period. Blocking will typically be given in "stage directions" such as "stage right" (SR) or "downstage" (DS). Stage right and left are the actors' right and left when they are onstage facing the audience. Downstage is closer to the audience, upstage is farther away. Always write your blocking in your script and draw small diagrams of furniture and set pieces you'll be working around.
Never say the word "Macbeth" in a theater; the play is supposedly cursed and brings bad luck to a performance or rehearsal. Instead, call it "The Scottish Play" or "Mackers." The phrase "good luck" is also frowned upon in the theater community as some sort of curse; instead actors say "Break a leg!" These theatrical superstitions are mostly for fun, but it is considered a faux pas to use these phrases in a theater, and doing so may reveal how uninitiated you are to your fellow performers.