In general, anyone who wants to become an environmentalist will already have a passion for, and an interest in, the natural world. This can be something as simple as an enjoyment of hiking, camping, or other outdoor activities. Alternatively, it could mean getting a background in environmental science, and working to develop new ways to help protect the environment. The type of environmentalist you want to become will affect the path you take.
Any type of enjoyment of nature represents an opportunity to learn more about the world around you, and to share that knowledge with others. For instance, if you enjoy camping or hiking, you might begin researching more eco-friendly ways to do certain things while you are doing these activities; you might then look for more eco-friendly ways to do everyday activities, such as conserving water or turning off the lights. Even these simple steps put you on the road to becoming an environmentalist.
On the other hand, you could choose to become an environmentalist professionally, and pursue a career involving research, conservation, or education. Any of these areas will require a degree in a related field, and an aptitude for scientific work. One other professional way to become an environmentalist might mean becoming a political activist, or lobbyist, and trying to affect positive change through new environmental laws and policies.
If you own land, there are other ways to become an environmentalist as well. Placing a conservation easement on a piece of land, ensuring that it will be protected from development in perpetuity, is one way to really make a lasting difference. Of course, even if you do not own property, there are still opportunities to become a steward of the land. You might volunteer for clean-up days, for example, or if you have a particular set of environmental knowledge -- such as the ability to identify many different types of plants or animals -- you might share this information with others on nature walks.
These are just a few ways to become an environmentalist without making a career out of it. There are plenty of environmental careers, from research and consulting, to land conservation and education, all of which will allow you to make your passion for the environment part of your career. Generally, becoming an environmentalist, whether you choose to make a career of it or not, will become an integral part of your life, and you will constantly be finding new ways to behave in a more environmentally friendly, sustainable way, and to encourage others to do the same.