First of all by stopping buying from all the multinationals who clearly contribute to pollution. Choosing what to buy at the supermarket is surely a good thing to start, in order some multinationals will start to change their production processes.
Plant more trees If you don't have land on your own, start making a garden in your apartment balcony Not long back, I read a post about a lady carrying different types of seeds which she can get(papaya, mango, jackfruit etc.) and throw them to vacant lands wherever she travels Fruit bearing trees consume more of carbon dioxide than flowering plants
Continue to raise the standard of living through establishing property rights and expanding capitalism. People take care of what they own, and they start to care about the environment around them when it is not part of the common collective. When you are starving and desperate, the wilderness is a source of food and firewood. When you are middle class, you can appreciate the wilderness in its natural state and afford to say no, don't cut it down, let it be.