WikiProjects are where you can go to find information about which properties to use to describe an item. You will notice that some WikiProjects have a lot of documentation about data models, where other projects have very little. Creating data models is a great way to teach new editors how to properly model items and help evaluate item quality.
In order to create a data model, you need to be familiar with properties that are relevant to that particular topic. Visiting item pages within that topic is a great way to see which properties an item might use. Here is an example of common properties from WikiProject: Medicine, grouped by popular topics.
Be sure to provide descriptions for the models you create so other users can follow your logic.
In the process of creating a data model, you may also discover that an entire property is missing and does not yet exist on Wikidata. You may propose a property. Property proposals go through a community approval process to ensure the property doesn't already exist yet, the property contributes to Wikidata in a meaningful way, and the property is formatted correctly.
Remember: Anyone can create a data model, so if you see that a model is missing, feel free to propose one.