It is a good idea to buy your tickets in advance online or by phone. For more information about how to buy tickets, see this page. The general day ticket is valid for the morning or afternoon and includes entrance to the Alcazaba, the Nasrid Palaces and the Generalife. This way you have more choice and you will be able to indicate the time you want to enter the Nasrid palaces.
Make sure you get into the queue for the Nasrid Palaces on time. If you are late they probably won’t let you in !!!!!!!!!