Whether you are wanting to renew your ticket or simply purchase one for the first time, I would encourage you to visit the official website as their is full explinations and instructions about how you should go about doing whatever your planning to do.
Please note that the club have announced the all renewals should be made by 1st July if you wish to reserve a specific seat (i.e. The one you posessed last season)
Discounts are available to all buyers until the 31st May so either get down to the stadium or visit the official site for an application form.