LC call numbers A to HF are on the 2nd floor, HG to Z on the 3rd floor. If the book's location is "Oversized Stacks", you will find it at the north end of the stacks on the appropriate floor for its call number. Call numbers that include "Ref" are kept on the first floor.
Use the guides posted on the stacks for general location, then look at the letter(s) and then the numbers.
Use this webpage for information on how to read a Library of Congress Call number.