I don't have my books in front of me, but I do recall that there are a number of examples in the GM's book, in the monster section. Giant Spider, Medusa, Serpentfolk, Snake etc. all had various examples. I'll check again when I get home and see if there was any others.
I'm not sure how it works in the beginner's box, but in the full version of the game, this webpage has a very good description (it's all the way at the bottom, in a field titled, "How Do Poisons Work?").
The game's developers wrote a rather humorous article explaining how poison works called "I Drank What!?" It is rife with explanation and examples.
Poison is much simpler in the Beginner Box. As Jim said, check the giant spider as an example of how it works.