The number one thing you can do to have a positive impact on the environment is don't procreate. That's right. Don't breed. I have gotten arguments that it's ok to have lots of children because we can feed a lot more people than there are now and there is plenty of space in the United States to live. Feeding people and living space aren't the reasons not to have offspring though. It is waste. Every human being creates waste. Also, having a child means that child will likely grow up and have kids of their own thus damaging the environment even more. I would argue that a person that drives an SUV, eats meat, litters, doesn't recycle, but doesn't have children, pollutes less than a person who takes public transportation, is a vegan, recycles, conserves energy, and does have children.
Ok. So let's think about how much waste having a child will create. Are you going to use disposable diapers for your child? Well disposable diapers are really, really bad for the environment. You have to feed the child so you have to factor in the amount of energy it takes to grow and harvest the food. You have to take the child for doctor's visits so that means you use energy transporting it. If it's winter, you're not going to have the heat turned down really low if you have an infant are you? Nope. You will turn the heat up. Child gets older. Has to go to school. Might take the school bus. You might drive them. Maybe you're lucky enough to live within walking distance. The child will use textbooks. So there are a few trees chopped down right there. Your kid will have friends I hope. Which means energy traveling to friend's houses and birthday parties and other get togethers. Taking your kid to baseball/soccer/football/etc practice. The list goes on and on. When the child comes of age will they get a car? Probably. Will they have children of their own? Well if they do you can double the amount of pollution. If they have a few then that means even more waste is created and released into the environment. By having kids, you are actually harming the environment even after you are dead and long gone.
Now does this mean that I think people that have children are bad? No. It is natural for people to have children. It is kinda hard to go against the prime directive of life which is to procreate and pass on your DNA. It's not that having children is bad in and of itself. Humans are a part of nature. However, nature needs balance in order for life to flourish. There are too many people on the earth. The earth's ecosystem can not compensate for the massive amount of waste products 6 billion people create. This is something we should be thinking about. When you see someone who doesn't have children and doesn't plan on having children except by adoption, then you should know that person is a great environmentalist. Even if that person drives an SUV, they are still doing better than the average person is for the environment.
This is one of the reasons I advocate adoption. By not procreating you are aiding the environment more than any other action or rather inaction as the case may be. Adopt a child, raise it, teach it good values. Maybe they will choose to adopt instead of breeding when they grow up.