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!!!