An 'environmentalist' isn't really a job, just like a 'feminist' isn't really--it's more about someone's values.
However, an environmentalist could be an ecologist, toxicologist, engineer, chemist, program manager, sustainability director, product designer, professor, philanthropist, consultant, etc. Every single industry has a need for someone who understands the environment and how everything we do depends on and is affected by it.
I'm an 'environmentalist' at heart, but I studied ecology, writing, and sustainable development. Now I help an IT company drive sustainable development initiatives, from designing more energy-efficient, recyclable products, to transitioning to a 'zero waste,' circular economy, to holding manufacturing suppliers accountable in reducing greenhouse gas emissions, and more! I have peers from college who own businesses, consult businesses and residences on energy efficiency, certify buildings as sustainable, advise on science-based state and federal environmental policies, etc.
I recommend taking a class either at university or a free online class on environmental science and pay attention to what interests you--there's no shortage of paths, careers and focuses.