The easiest way to locate a biography is to do a Subject search in The CAT. Enter the name of the person as a subject. For autobiographies do an Author search.
Many people are not famous enough to have entire books devoted to them (or you may not want to read an entire book!) In these cases your best bet is to consult a biographical dictionary or encyclopedia. Typically these sources provide a brief essay (a paragraph to a few pages) summarizing the individual's life and accomplishments. The library has hundreds, if not thousands, of these sources. The ones below will get you started.
The library has acquired materials by and about prominent Pennsylvanians including Albert Gallatin, Joseph Priestley, Mary Grew, and William Penn; To determine if the library has sources for the person you are interested in try a keyword search in CAT using their name as author.
Many personal papers are NOT published and can only be accessed at the library or archive holding the original documents. The Useful Web Sites section of this guide contains links to libraries focusing on Pennsylvania History.