Simply find the book/s you want and take them to staff at the library counter along with your library card (if you're not already a member, you'll need to join the library to get a card). They will issue your book/s, including a receipt that states the date you need to return the book/s.
If your book is due back but you're not quite finished, you can renew it, as long as no one has placed a hold for it - renewals will be for the same length as the borrowing period (e.g. if you borrowed the book for two weeks, you can renew it for two weeks).
To renew a book, log in to the library catalogue using your library card, click on 'My Account', then 'Renew My Materials', then select the book/s you want to renew.
You can also phone or visit the library to renew your books.