Pretty easy, really.

Sustainability is a pretty simple concept.
Can something (some practice or some action) be done forever, or for a very long time, or at least for as long as it's necessary? That's pretty much it. In order for something to last, it probably has to be self-sustaining, which means it must be replenished or it must be replaced, preferably the former.
We are consumers. That is, we consume. We must eat and drink in order to survive. When we are behaving in a sustainable manner we then, in turn generate or produce something that some other organism needs to survive. Perhaps our waste is employed in the fertilization and cultivation of some produce, or perhaps we become food for some predator. At least in that scenario, we have a purpose.
The most important thing we can do is to be aware and to think about the consequences of the things we do. This is a practice that has fallen so far out of popularity that it has received it's "free-fall skydivers" certification.

We have come to become ruled by convenience. That is the most important thing anyone can seem to think of. Let's build stuff to save time. To measure time. To avoid walking. To avoid working. People just throw stuff away because it's easy. It doesn't matter if there's some use left in it, or some raw materials that can be extracted, or the fact that it just looks bad sitting there where you threw it. And using stuff up is even worse. We are trying to have a society here and why don't you think everyone should get a fair share?

I notice that no matter how thoughtless someone gets, they still manage to get up when they have to defecate at night. It's obvious why.

So, now apply that same logic to everything that you use. Should I bother to get up, or just sit here in my own filth? Same applies to every thing that we do in life. Just think. That's all.
Oh, and then do something about it.
Baby steps...