Koh Tao, a tropical paradise, shouldn't we keep it that way? There are over a 100 000 visitors per year, who obviously have a severe impact on the eco-system. Unfortunately there are no garbage disposal facilities, what can't be burned, has to be taken away. There is no department to clean roads or the open landscape.
So, it is everybody's responsibility to help keep this problem as small as possible.
Here is what you could do, to make a difference.
Watching bad examples, does not mean you have to follow them, better give a good example to others.
There are containers for bottles and cans, use them.
Rubbish bins, even difficult to find, are always a better option than dumping your waste in the open landscape.
When going trekking, don't leave a trail of garbage and empty water bottles behind.          
Try to avoid using the plastic bags that will be given out for any little item you purchase, bring your own bag instead.
Cigarette butts need ages to decompose & the ashes spoil the beach, so use ashtrays or empty cans or such.
Water is scarce, especially during the hot season, don't waste it, don't leave it running, especially during a shower.
Electricity, supplied by generators, is very expensive, be economical, switch off the lights when leaving your room.
Don't throw paper or other things into the toilets, the pipes are easily clogged, but very difficult to clear.    

The sea is naturally at higher risk because pollution is not as instantly visible as it is on the land. Imagine that yearly cleaning events organized by the dive schools retrieve tons of garbage from the bottom of the sea.

First rule:  Don't throw anything at all into the sea!

    Second rule:  Just look, don't touch!                                  

Marine life is very sensitive & easily harmed, some is poisonous and some will just ... bite!
Corals are very slow growing, but fragile and easily destroyed, often by accidental contact with the scuba gear.
Dead corals and shells are very important for the bio-cycle, so every time you want to pick up a souvenir, consider the impact of thousands of tourists doing the same must have. Besides most underwater beauty fades away quickly when taken out of its natural habitat, a photo or video is always the better and longer lasting choice.
All in all, the basic rule should be to simply to leave this fantastic place same as you like to find it:  Unspoiled!