This may be a good starting point:
Its great that you want to be a force of betterment in the world. There are many different kinds of ecological activists, but whats important is that you become the one where your daily life consists of activities you enjoy. Otherwise, you will get burned out soon.
The most straightforward way is to look up NGOs (others have posted links) and try to see if you can get a job with the one that seems exciting to you. Change is always brought about by acting in groups, so joining or creating an organization is the best way.
My answer to your question 1 is as follows. You are probably too small to make meaningful impact on a global scale - no one is. This is a task for governments. But you can make impact on a local scale. People often forget that there are many many things you can do at the city level to create sustainable living patterns. You can join or create organizations that are trying to fix your local problems. One interesting thing that someone with your background could do is to use art to drive social change. I know a excellent organization in my city that does community theater activities to educate and impact people, and you can definitely see the positive effect they have in the communities and areas they work in.
Be the change you want to see. You can only control your own actions. There's no futility in that.
Look towards yourself instead of institutions.
Marta, I need to be blunt here. And I realize that sometimes being direct with a female these days is politically incorrect. So be it.
Your background is such that you are far more likely to be used by climate change zealots. Which means you are likely to unwittingly engage in spreading lies and non-solutions.
If you really want to help humanity in some of these domains you have to have enough of a scientific background to be able to understand the subject well as well as to separate science from religion.
The good news is: You can acquire this knowledge. If you are serious about this, shift your focus towards studying science. The rest should follow naturally due to your passion.
FIRST ACTION: Obstacles, identify everything that is standing in the way.
Obstacles become the raw material for achieving your goals.
ON this subject, Dan Sullivan is brilliant > https://www.33voices.com/interviews/the-way-to-defeat-your-o...
Here is a startup incubator that focuses on activist companies. Maybe one of their startups would be a good place to work?
https://www.highergroundlabs.com/about-us
You could also consider running for office. There are several organizations now helping novices run for public office from local level to national.
Climatic, there is nothing you can do. The vast majority of pollution comes from corporations, not individuals. Try to rally a change in laws, but don't focus your efforts on changing individual people's habits, it's a drop in the bucket.
Marta, You're looking outside yourself for too many answers.
Just live life and have experiences with the thought that "I'm going to let life teach me about myself". You don't need more schooling but you do need to put in work at hobbies, a job (you don't need to love the first thing that comes around. There's always another job), personal relationships on all levels (friendly, acquaintance and intimate).
If the above is too ethereal for you, find solace that everyone's lost at 25 (and many are beyond that). You just don't have enough time in to LIFE. Keep doing things until you find the VERY FEW that cause you to find yourself.
Good luck.