This question has two main parts.
First let's define overpopulation and underpopulation. Then ask yourself, what causes each of these states? And what is the result?
Create a table or a spider to organise all your ideas.
Go to YouTube and explore population videos. Not all the videos agree that there is a world problem at all.
See how many different points of view, and different arguments you can find. For example, the Population 101 series starts from a different point of view to the one we normally see.
This document might also be helpful, remind me about the activities at the end to try.
Insight Zambia, BBC news video here.
You can see that population isn't just about graphs. It's politics, ecology, your rights, your responsibilities, the lives and decisions of other people and the flows and changes of social groups. Not much then, eh? Try an article by Jonathon Porritt.
What would you say in argument to a person who asserts that 'to have more than two children is irresponsible'?