Identify subjects (ofnesses, isnesses, aboutnesses, etc.) to include in your representations of documents in your domain.
Survey some existing subject languages. Try to identify how other people have categorized items in your domain, and what kinds of labels they have given the categories.
Ideas for sources to consult:
Thesauri, term lists, ontologies, etc. from the Useful Resources Wiki Page or other sources
Category labels used to organize websites about your domain, stores selling object in your domain, etc.
Taxonomies, glossaries, or indexes from a books about objects in your domain. Reference sources are very good for this.
Compile a list of potentially useful vocabularies. Do this on the wiki.
This does not have to be a formal list or bibliography, but I want to see that you are finding some things to work with/from. Examples of what kind of information you might include:
Short lists of terms used in website navigation schemes