Will Man's selfish acts bring about the Earth’s doom?

Yes they will. Man has been very cruel to the earth and they do not see the need to conserve it. They are not future seeing as if we continue doing what we are to the earth, the future generations will suffer in ways such as : A shortage in oxygen as it is constantly depleting as man keep cutting down precious trees for resources or for land space, and in a few years time, the temperature of the earth will spike and the people in future generations will suffer from access heat.

What can we do to save the Earth?

We can save the earth by using less plastic bags and cut down on fuel emissions. One thing i feel the government should do is to conduct a check on everyone's cars and lorries and test them. If they exceed a certain amount of fuel emission, they should warn the people owning the vehicle to change it.
What are some of the problems that we can realistically and effectively be engaged in?

We can start by recycling our used things.