I have been acting for 30 years and most of my training comes from working on plays with many different directors and other performers. There are a few basic, basics that will help anyone, but the number one piece of advice I think I have consistently been given is: DONT BE AFRAID TO MAKE A FOOL OF YOURSELF!!
Step 1: Pay Attention!!!
So "assuming" that you will be acting with other people (monologues are a whole different thing) the next most important thing is to LISTEN!! Acting is RE-Acting. Pay attention to what the other characters are saying. You are most likely to be responding to what someone else is saying, even if you aren't answering a question, you are having a conversation. This is important in life too. Don't just wait to say your lines at the right time, although when the time comes to loose the script, you want to pay attention to your cues. This is when you know it is time for your line. But what if the other person flubs their line or paraphrases, leaves something out, etc - you need to understand what the conversation is about, so PAY ATTENTION!!!
Step 2: Take It Slow!
Now that you are listening to what others are saying, think about what YOU are saying and SLOW DOWN. We have a tendency to talk fast when nervous and you know that's going to happen if you are a new actor. A little favorite mind trick I like to call up is to NOT use the word "Nervous" try thinking "Excited!" instead. This feels more positive and helps me use that energy in a good way.
Step 3: Walk & Talk
So you are paying attention, slowing down, excited and ready to walk and talk at the same time. During rehearsals the director will be "blocking" the play. This means they will decide where everyone is on the stage at any given time. LISTEN to the DIRECTOR They will tell you where the entrances, exits and all the movement in between will be. Be patient. This can be tedious, BUT it will also help you remember your lines when the time comes. Connecting the words and movement sets them in your body and makes it easier to remember what you will do next. Eventually (this is the goal) it will become second nature. That's what rehearsals are for! "Learn your lines and Try not to bump into the furniture. " - Spencer Tracy (That's him in the Photo!!)
Step 4: Be Nice!
BE NICE TO EVERYONE!! You are all in the same boat, working towards the same goal. Help each other. Don't be a DIVA!!!! AND Have Fun!!!