I'm not sure what my first one small thing was. Probably, it was choosing not to buy something that I didn't really need. And from there it blossomed out into a whole way of life where we are trying to reduce the impact we are having on the world as a family. Choosing to change one small thing at a time has worked well for us, because we know from personal experience that trying to change too much at once doesn't seem to work -- and gets really overwhelming. Even tiny contributions to the well being of the world are contributions.
Starting small is simple:
Get a library card... and eventually, stop buying books period, and save some trees.
Walk or catch the bus to work... and eventually, make it a habit and save the earth from some fossil fuel emissions.
Grow a few carrots... and eventually, a small vegetable patch or container of vittles, so you can eat local.
Cook one vegetarian meal... and eventually, cut reliance on meat products and save the planet all sorts of resources related to animal production.
Carry cloth bags for shopping, and eventually, reduce use of plastic in other areas of life, too.
There are dozens and dozens of other ways to make a difference. The trick is to make a start.
What is/was your start?
It is always good for individuals, communities, and indeed nations, to remember that their present situation is a result of thousands of gestures of love or hate that came before. This obliges us to remember that the community of tomorrow is being born of our fidelity to the present. We discover that we are at the same time very insignificant and very important because each of our actions is preparing the humanity of tomorrow; it is a tiny contribution to the huge and glorious final humanity.