Explore this list of WikiProjects to familiarize yourself with the various projects on Wikidata. You'll find data models for every subject-area on this page.
Join a WikiProject that interests you.
Evaluate a data model and suggest adding some properties that may be missing from it. Don't forget that you can find data models on WikiProjects. In the event they don't exist, you will have to create them yourself.
Propose a property that doesn't exist on Wikidata yet. Think about how to describe important concepts in your field. Is there something that expresses important connections between these concepts and others? When you try to add more information to items are you frustrated because a current property is not the best fit? This could be a candidate for a new property.
Propose an entire data model for a concept or thing that does not exist yet. The more well-documented data models Wikidata has, the easier it will be for editors to contribute in a meaningful way and the easier it will be for users to understand how Wikidata works.