Once you have done a search in the Catalog and found the title you want, click on the green "place hold" box.  The pick-up location will default to the library which issued your library card to you.  If you would rather pick up your item at a different library, choose it from the list, then click on the brown "place hold" box.  The next screen will tell you that the hold has been placed and a staff member will contact you when your item is available for pickup.  Click on "OK".