Do not go for a monologue just because it is your favourite. May be your age is not appropriate for performing it. Keep in mind not to choose monologues with similar tones as these will stereotype you. Do not underestimate yourself but be honest because you can’t perform every role. If you choose an inappropriate monologue, the director will think that you have unrealistic expectations and that you are not capable. There are many options from where you can get hold of good monologues: libraries, online, original plays published etc. Reading the original play is very important so that you understand the character well.
When you finally choose a monologue, break it down into segments. Many experienced actors do this with their lines. This way you can understand the meaning of every line separately. As there are many people competing for the same role, you will be better prepared. Attend an acting class and practice the character that you will be playing. Work on the character until you feel you have a full grasp on it. It will ensure that you are connected to the character and will be able to give a natural performance.
Practice makes a man perfect. Rehearse your dialogues in front of trusted friends and family. Ask opinion from your teachers and elders who have experience. Ask for feedback and keep on improving your performance. This will also increase your confidence and will make you comfortable in front of people. Make sure you do not give pauses as this will have negative impact on your performance. When you get feedback from people, do not take it negatively. Accept the feedback and try to work on it. Analyze the feedback objectively and then work on it. Make sure that you keep practicing so that you stay in form.