First, great to see you wanting to get involved.
You might start by looking at what is out there. There are many worthwhile organizations in the environmental space who need the help (dollars, time and more). Ultimately, If you already have some organizations in mind, then skip the next part. If you don't, start with what you are passionate about - and ask yourself some tough questions. What do you care about? Animals? Wild places? The health of the world? And then get more specific: do you like cuddly animals and if so, which ones and why? Is the ocean important to you? Depending on what kinds of things you are passionate about, you may also want to think about "where" you want to have an impact. For example, I'm very passionate about the Yucatan, so I support a group there. You may find that there are groups already doing what you want to do - and it might benefit the animal/environment more by joining forces and supporting the existing group. Do your research and make sure the group matches what you want to do. It makes a big difference.
If you look at supporting an existing group, the first thing is to find out what they need. Some are good about advertising that on their web sites. Sometimes it is just money, but maybe there are other things you can do that are just as important, like volunteering. Remember: you're trying to help them, so the best place to start is by finding out what they need.
Now as for starting your own, there are some legal hoops. Others might be better positioned to help, but given some of the charities that exist out there, I have to assume that it's not the hardest thing in the world... What you want to do when starting an organization is to figure out some basic things:
What is your vision for success? if you "succeed" in doing what you want to do, what will the world look like? What will have changed?
Use that information to then figure out what your mission should be: why do you exist? IE because we want to make ^^ happen.
How are you going to do it? What kinds of goals are you going to set? what are the things you will do and what will you not do?
All those other questions from above apply, too: what animal or activity? where? etc.
With that, you can then figure out the "how" behind all of this. Will you just raise money to support other groups via grants? Will you raise money to take a direct action? Will that action be performed by you? By you and a staff? By someone else entirely? What do you need to succeed?
Armed with some of that information, you'll want to start putting together a plan. And that's where fundraising becomes ONE part of it. You would want to set up a mechanism for securely accepting money. Causes.com is a good place to start for that, but you'll need something yourself. Whoever can help you with setting up the nonprofit in the first place should advise you about how to set up a bank account, get your tax-exempt status, etc.
If all of that is good to go, you now need to tap into things like causes.com as a fundraising platform. Social media will be your friend and you'll want to use it to connect with people who have similar interests. And not just follow/like - you'll want to become part of the conversation.
Talk to friends, too. Get them to help. They could volunteer some time, or donate money. If you have an existing event set up (say a Sunday afternoon football party next weekend...) you might consider asking everyone to pull out their phones and share a story and your URL or make a donation or something else that could help.
Hope that helps a little. Good luck!