Poetry is notoriously a bit difficult to shelf-browse for within the Library of Congress classification system. It groups fiction and poetry together, loosely organized by geography and century. Although poetry books will be scattered about the P section (see map below), there are a few particularly helpful call numbers ranges you might want to browse for English language poetry:
Have a poet in mind? Search the library catalog with the author's name. If their name is common or the results are not relevant, you might want to add AND poetry as an additional keyword.
Browsing by theme? Search the library catalog using "poetry" as one of your keywords along with a topic, like "nature AND poetry" or "healing AND poetry"
Looking for a wide range or a variety or authors/eras? Poetry anthologies can be a great place to start.

Suggested search terms might include a genre, medium, time period, poet, or subject area. They may also include a combination of categories. For example:
Experiment with your search terms to find a bigger variety of resources.
For example, enter "Sylvia Plath" into one box and "biography" into a second box to get more results.
Boolean Operators