One of the best things you can do when you write for a tabletop game is establish your gameworld. It was something that I lovingly and painstakingly did when I first started using homebrew materials in a serious way--which was even slightly before I started running serious games. That's the thing: to run the game you want, you need the world that matches. That's why I spend so much time here on the site addressing worldbuilding.
I openly acknowledge that much of my worldbuilding is cultural. That's partially because I am very interested in cultural affairs in real life. But it's also because in any campaign that includes serious storytelling or open space for roleplaying is going to address culture quite often. I think that portrayals of culture make the world seem distinct from ours, and they can be exciting to learn about for players. So with that in mind, I want to build a guide to stereotypes. By this, I mean that I want to explain what people generally assume about various groups within the gameworld. As in our world, stereotypes are not necessarily true, though, so don't take a stereotype that a particular race is dishonest as an actual statement from the GM that it's true. In fact, my belief in real life is that most stereotypes are false and that most people are pretty similar in the ways that matter most. So these stereotypes are simply beliefs that are held about fictional people by fictional people.
The following is a guide to stereotypes by D&D race, class, and religion.