Searching by subject can be tricky, because the Library of Congress has very specific subject headings it uses and if you don't know the correct term for your topic, it won't work for a subject search. For example, chaos theory is not an official subject; instead, it's "Chaotic behavior in systems."
However, once you have done an initial search (keyword or other), you now have resources to start your subject search. Use the links on the sidebar (pictured above) to do subject searches. Clicking on a link will take you to items with both that subject AND your initial keyword search. Alternatively, typing the topic subject into the catalog search bar and changing the field to subject could get you a larger variety of results.
Click on a item's title to view the item's record.
The item record shows valuable information about an item. Besides author, publication information, and date, the record tells you what collection the item is catalogued in, and where to look for it in the library (see Finding Books for information on how to read call numbers). Lastly, the record shows if the item is available, and if not, when it is due back in the library. If an item is unavailable, you can click the "Place Hold" link on the upper right, and the library will reserve it for you as soon as it is returned. To place holds, you will need to sign in to your library account (see My Account for more information).
One useful feature for research is the subject area of the record, below "Record Details." This provides you with a link to a subject search related to the item you are viewing. See the image below for an example of where to look on an item record for this useful classification.