Let's say you are a member of drama club, and your club is going do a play next month in front of the students. How would you prepare for the play? As we are not genius at acting like some famous actors, we do need a thorough preparation before the play. So, if you don't want to become a laughing stock in front of your school mates, you have to go through three main stages below.
First of all, read the whole scripts as much as possible, at least three times. Sometimes, people think the role they are going to play is the only thing they have to know about. However, you have to know not only about the role you are going to play, but also the whole story line, relationships between different characters, background information, and so on. You can learn all these by carefully reading the script. But reading only once would not be enough at all. So read again, and again, and again even till you almost memorize the whole script and can explain in detail about the story whenever you are asked.
After reading the script, the next step is to study and analysis your character. As an actor, you have to be fully responsible for your own role and make yourself impossible to be replaced by others. So you have to analysis your role and know well about it than anyone else. One way for this is by becoming that character even when you are not playing. When you are having lunch, brushing your teeth, taking bus to school or even before getting into sleep, think about the role. As you think about it again and again, you will be familiar with your role and realize many aspects of her.
Finally, it is time to apply what you've learned to the actual practice. It is pretty much different between "knowing" the character and "acting" the character, so that's why practicing is essential before you play in front of the audience. During the rehearsals, help each other with other actors and ask them what they think about your acting. Then, they would probably give you good feedbacks so you can reflect them to your playing. Also, don't forget to check if all the music and other stage effects work properly so there is no mistake on the actual stage.
After finishing all these three steps; reading the scripts, analyzing your character and the rehearsals, you will feel much more confident than before. Although it seems too much for you to do, they are the steps that will help you to become a better actor or actress and also make the play much better.