There are 3 choices
1. Get a guided Tour:
Getting Alhambra tickets is a complicated process with a lot to understand. If you don't want to have all the hassle of understanding about the tickets and what the Nasrid Palaces are and when to get in the queue and where to go then you can take the easy route and just get a guided tour. Someone else will take all the pain away. There are no decisions to make. This is by far the most expensive.
There are 2 guided tour links herehttp://granadainfo.com/alhambra/eng_guided_tours.htm2. Get the tickets on ticketmaster.
Get them as far in advance as possible. You may have to spend 15 - 20 minutes on internet buying the tickets and understanding the implications. About 60% of posts on this forum are about buying tickets to the Alhambra.
This is much cheaper and is better if you like making your own decisions and don't like spending too much money.
This site has very informative info about the choiceshttp://granadainfo.com/alhambra/3. Get the tickets on ticketmaster but get an audio guide
BE CAREFUL - The audio guide has been suspended we don't know if it is coming back.
You can hire a little mp3 player with a commentary to give you more info.