Omeka uses the Dublin Core Data Standard for structuring the metadata for items. Familiarize yourself with the basic elements of Dublin Core, including guidelines for how to define and format the information contained in each element and looking at examples.
It's also helpful to take a step back and understand why it's important to define what information you're providing to people when you describe your collection. See "Access Points" (pp. xxii-xxiv, or pp. 23-25 of the pdf) in Describing Archives: A Content Standard (DACS). Focus on defining a consistent way of approaching the six categories of access points.
Dublin Core is a data structure standard whereas DACS is a data content standard: DACS provides more in-depth guidance for how to describe your collection and items in the collection. It covers a lot of ground, so don't get lost in the weeds--I find it most useful in terms of the examples for elements like Title, Scope and Content.
Before beginning cataloging, it can be helpful to document the required elements you will be using to describe your items and collection, and what standards (formatting, vocabulary) you will be using for each of those elements. This is especially important if there are others contributing to cataloging and description. An example of such a document is one I created for the Clark Center art collection--it's both a "crosswalk" because I was mapping to several different asset management systems/structures as well as a dictionary. Take it as an example only--some of the standards used have since been revised.
Getty Thesaurus of Geographic Names can be used for place names.
Virtual International Authority File - Use for subjects/concepts, names, and locations.
Do a few searches for terms (names, places, object names) in these vocabularies to get a sense of what's contained in them, and how the terms are defined.
Once you have a sense of what your collection is about, it's good for consistency's sake to create a short list--a pick list, if you will--of terms you'll use, especially for metadata elements like
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