Although Tibia is just a game, it is an online game, and one of the unique traits of online RPGs is that you can interact with other players, to form a community on your server. Thus, since you aren't playing against automated computers, you should respect others and avoid being discriminative or offensive to them. Being part of the community has many benefits; it generally makes playing more fun, allows you to engage in roleplay activities, and you may even make a few friends who will likely last longer than Tibia will.
Treat other players as you would like to be treated by them!
Also read the post by GM Freehugs, about how even a small act can make a big difference. You can read it here.