It's likely that at some point you will be asked for your Reader's code (also known as a University barcode). If you are uncertain about what this is, it's a 5-digit code which always begins with the letter 'V' used to uniquely identify you as a library patron.
There are two places to find this code, depending on whether you are a member of the university or not. For members of the University with a blue University card the code is found on the reverse of your card, as seen in the image below:
For other library users the code is found on the front of your card, as seen below: 
See also: How do I log in to my library account on iDiscover?, The catalogue says I need to 'order' a UL book. What does that mean?