Always do reading in secondary sources before looking for primary sources. Often times, information in secondary sources will lead you to a primary source. For example, an article in a reference book might reference a speech or letter written by the person you are researching.
Primary Sources in the OWHL Catalog
Do a keyword search for your topic and include the words primary source in your search string. For example: when looking for primary sources about Thomas Jefferson you should type Thomas Jefferson primary source into the search box.