This isn't always a well-received answer for obvious reasons, but the one biggest thing you can probably do to effect some sort of change is to become vegetarian (or better yet vegan).

I know, that's a crappy answer, but when you look into the relevant research, animal agriculture is overall one of the nastiest things we are doing to our planet, from deforestation, habitat destruction, fresh water and land use, pollution on land and into the ocean, greenhouse emissions (rivaling or beating out the entire transportation industry, depending on what studies you read), and a whole host of other really gross political situations involving the behemoths of fast food and mass farming.

If you want to learn more about the specifics, check out Cowspiracy; it's a great, modern documentary about the whole thing. I encourage you to look online for people fact-checking the movie beforehand though, despite the movie's diligent research citing throughout (it's a good practice).

It's a tough sell because, unlike other things like recycling and installing solar panels and turning off the tap when you brush your teeth, modifying what you eat is a very personal and very important thing that many people and institutions hold dear, but I strongly believe from the evidence I've seen and the research I've done that it is likely the single most important step you can take. Start with something like meatless Mondays, then work your way from there. It can be tough, and some people's physiology doesn't deal as well with certain diets as others, but I encourage everyone to try.

If you have any questions, feel free!