The Alhambra is the most famous place to visit in Granada. Please have in mind that most of the time it is necessary to buy tickets online in advance, most of the time weeks or even months.
To buy tickets, go to the official website : Alhambra tickets
If the tickets are sold out there, try on this website : Alhambra ticketsThey offer all kind of tours with or without guide. Pay attention to the details of the tour. Many of them only visit parts of the monument (not the Nazari palaces).