[Koha] newbie needs help importing records

We have just downloaded and installed koha for use in our small Catholic school library. We have no current computer records for our books (about 4000 items). I am not a techie or a librarian, but am trying to get this set up. I could really use some help in how to start importing MARC records via a z39.50 server. I am lost in all of the manual directions about MARC identifier fields, etc. In my perfect world, I would like to take a book off of the shelf, search by title in the Library of Congress or other database, import the MARC record, stick a bar code sticker in the book and scan it. Can anyone help me access some basic directions for this or get me started? Thanks so much Shelley