Everyday we affect our environments, positively or negatively. How? Very easy, the exact same way I am doing right now. We have contact with people, be they people we know or strangers. We do things, say words, type on our computers. Easily we type long messages, or chat with our friends on the phone. Easily we drive in our cars or wander in and out of shops.
But, do we really pause and think, "What effect am I having?"
Take these two sentences, each consisting of three words.
1) I love you.
2) I hate you.
The two sentences have not only the exact amount of words, but the exact amount of letters. Yet the implications and effect they have on the one who hears them are immense. One sentence can make a someone the happiest person in the world. The other sentence can forever tear someone's heart apart.
Not only what we do has an effect on others, but it has an effect on ourselves. Everything thing we do is recorded, waiting to be shown to us on that Day when all secrets will be told. The time when you made your mother proud, the time when you embarassed your classmate, the time when you wrote a soothing reply on a forum, the time you wrongly accused a person... even the time you kicked a rock to the side of the road.
So think and then act. Allah has not given us a brain so that we do not make use of it. Use it for the pleasure of Allah. When you are being kind to others, when you are taking care not to cause others pain, then that brings the pleasure of Allah. And when Allah is pleased with you, then there is nothing else in the world that you need or could wish for.