You will need to decide whether the field will use terms from an existing authority (e.g. AAT or Nomenclature) or from a set of terms you build yourself. It's often a combination of both. It's common to start out with a standard set of terms, and then add new terms to the set when you find something you need is missing.

The advantage of using an existing vocabulary is that there is a lot of work involved in developing your own, and it's a complex task. If someone has already created something that suits the scope of your collection, there's no need to reinvent the wheel. Also, your data is more accessible to people outside your own institution if you follow a widely used standard. If you have a collection which is quite focused, you may want to adopt part of an existing vocabulary, instead of the whole structure, e.g. one facet of the AAT. For a discussion on the merits of different sets of terms, see Museum Cataloguing Vocabulary Authorities?.

If you choose to build your own vocabulary, it's important to follow some simple rules, to stop your Lexicon from becoming tangled and to make sure it's clear who's in charge of maintaining the Lexicon.

If you have already entered terms in the field, but they have no structure, or not enough structure, or bad structure, you can gradually replace this with a standard authority or with a well-structured local system. (See The Tangled Lexicon)