There are many problems resulting from overpopulation. In light of this, I will list a few of them.
First, with the growth of population, the most obvious problem is the lack or resources. We need to keep in mind that there is a limited amount of commodities. We do not like to think about this, but it is a reality. There are limitation to things like: water, minerals, food and the like. Starvation and shortges are a possibility.
Second, with the growth of population, there are also limitations of other benefits. For example, how can we provide the modern benefits of things like medicine, health care, and housing?
Third, people create waste. With the growth of population, we need to worry about things like what will we do with all the waste that we produce.
These are just the tip of the iceberg. There are also problems in education, housing, and employment.