Right now, you are sitting at your desk, perusing this website with an eye towards increasing your knowledge about sustainability ...maybe you're even thinking about how to become more involved in making your self/neighborhood/community more sustainable ...but you just don't know where to start.
Look around you.
What sort of light bulbs are in your overhead lights? What temperature is your thermostat set to? Are there plastic bottles and aluminum cans thrown away in your trash can? What type of paper is in that tablet next to your phone? Can you see your coworkers' screensavers running? Can you hear a faucet dripping in the distance?
Think about your commute to or from work today.
How long were you in the car? Was your commute short enough that you could have walked or ridden your bicycle? Did you stop off to grab a bite to eat through the drive-thru? Did you wrinkle your nose at the exhaust smell coming from the car in front of you?
Think about the meals you've had today.
Do you know where the fruits and vegetables were grown? Was that apple organic? Do you know if your protein source is an endangered species? Did you use plastic or metal utensils?
Now, why are these questions important? Because they address just a few of the issues and actions that impact sustainability in some form or another.
Sustainability is, at its most basic, the capacity to endure. But more than that, it is the notion that everything we as humans need for our survival and well-being are dependent (either directly or indirectly) on our natural environment. Therefore, it is important for an individual/community/region to balance the environmental, economic, and social demands of any action that is undertaken in an effort to find that sweet spot that makes that action sustainable.

So...look around you again.
If you recycle those plastic bottles and aluminum cans, you reduce the amount of energy required to produce those items from virgin raw materials, which conserves natural resources such as fossil fuels, petroleum, and aluminum ore. That's working towards sustainability.
If you avoid the drive-thru and order your food at the counter, you reduce the time your car spends idling, which conserves gasoline and reduces emissions. That's working towards sustainability.
If you choose an organic, locally-grown apple, you reduce the amount of pesticides that wash into surrounding water bodies and limit the amount of travel time that apple had to undergo, which protects surface water resources, conserves gasoline, and reduces emissions. That's working towards sustainability.