The Alhambra is the most-visited monument in Spain... so, if you want to visit it (and you'll probably want) get your ticket as soon as possible.
All the information about visiting this Red Castle (that's the meaning of Alhambra) available at http://www.alhambra-patronato.es/index.php?id=8&L=2.
You can buy your tickets at Ticket Master. Tickets for september could be bought since June or JUly, as far as we can see. We will check it frequently to get you informed.
Finally, you can also buy the official guide of Alhambra for both iOS and Android.