Assuming it is a non-fiction book (school libraries rarely shelve fiction books by the Dewey numbers designated for fiction) you should be able to find it using a subject or subject keyword catalog search.
If there is a specific book but you don't remember title or author, then you can use the library catalog (which is usually online and accessible either from your home computer or at a terminal at the library) and enter a keyword that relates to its subject matter in the right field, which might be "subject" or "Keyword" depending on the catalog. If you remember any words in the title, you might also search the title field.
Libraries should also have a guide to the Dewey Decimal system...you don't have to memorize it. Look at this guide and determine the subject and corresponding number or ask the librarian or study the library map to find where that shelf might be located.
Dewey Decimal is the classification system of the library.
The Dewey Decimal Classification (DDC), or Dewey Decimal System, is a proprietary library classification system first published in the United States by Melvil Dewey in 1876.
know the ten main devesions of the Dewey decimal system
Astronomy is in the 520 section of the Dewey Decimal System.
Melville Dewey
The Dewey decimal number for Greek civilization is 938 in the Dewey Decimal Classification system.
Shakespeare is the only author to have his own Dewey Decimal number.
melvil dewey
Dewey decimal system
Dewey decimal letters refer to the letters at the beginning of a Dewey Decimal Classification number that represent different subject areas. For example, in the Dewey Decimal Classification system, the letter "D" refers to the general category of history and social sciences. These letters help organize library materials by subject.
The Dewey Decimal System and the Library of Congress Classification System.
The library uses the Dewey Decimal Classification system to allocate numbers to books.