Hello! I am hoping to be of some use to Wikipedia, however my coding techniques are not good yet, hence my lack of contribution. Must spend more time practicing. I made my first contribution in the Pro-Anorexia section after, honestly, coming across it and getting pissed off at the blatant advertising a specific site was making on it. I didn't believe it was ethical to have such sites linked so directly when they cause far much more harm than good.
If the sub-category page does not already exist, then you have to create it first (see the tip How to create a category). For example, after you create the Category:Apes by common name page, then you click edit and add [[Category:Animals by common name]] to the bottom of it.