Wow, great question! I think for me it would be one of two things, one technological and the other mental/emotional/sociological. In terms of technology, if we could quickly, carefully and successfully move away from fossil fuels to fully clean, renewable energy sources we’d have made a gigantic leap towards a more sustainable future. Climate change would slow down and countries could grow and flourish with much less impact on the planet. On the more human level, we need to learn to be less greedy. Lots of damage to the planet is driven by wanting stuff we don’t need – the more stuff we want, the more resources we take and the less sustainable we are; and also, the more other people want it too, so things just accelerate. If we could be happier with less, taking a bit of a decline in our wealth or lifestyle, we could make room to improve the lives of lots of other people on the planet too, and Earth would be able to cope better.
there’s a great infographic in this article: https://www.popsci.com/how-to-stop-climate-change#page-4 (scroll down ’til you see the green graph with clouds). The graph makes it pretty clear that using your car is probably the worst “normal” thing you can do – but I guess when you’re at school you might not have much control over your transport methods. Trying to eat less meat is one that perhaps you can do more easily. Meat is incredibly easy to cook right and still be yummy, but veggies can be delicious too – if they aren’t you probably just aren’t cooking it right! Try giving a sweet potato 5 minutes or so in the microwave (until it’s soft), slice it open like a jacket, and smother it in baked beans. Delicious, easy, and plant based!
It is hard just to pick one thing… We should definitely stop flying! But then most of the world doesn’t even fly so another thing people could do is not to have children. Think about the amount of carbon one person uses throughout their life… I know it’s not what people want to hear but as a species we ought to be aware we only have this one planet. Oli’s and Yo’s suggestions are also really good though and would be next on my list 🙂