Start small, clean your room, clean up your yard, then your neighborhood, then the community and so on.

Seriously, go down to the local river where people fish and leave their trash behind. Take some garbage bags and go clean up the riverbanks, or the park, get others involved.

It does not matter what it is or how small. Just go and do and don't try to save the world, just one piece of it at a time.

Edit: A trap we fall into is the desire to fix a global issue while ignoring the local issues we can change.