From the library catalog, you can place a hold on an item that is currently checked out by clicking on "Place Request". A hold prevents the current borrower from renewing the book. When it is returned, it will be placed on hold for you. Then the library will (based upon your account preferences) call, email, or text you to let you know when the item is available for pickup at the library.
Please note that any hold items not picked up within four days will be cancelled.