The best way to get immediate and guaranteed access tickets to Alhambra (jump over the lines), is to buy the tickets online here.
In case they were sold out, you may buy the Granada card here. This card gives you access to all sites and monuments of the city of Granada, including Alhambra (Nasrid palaces, Gardens, Generalife palace and gardens, Alcazaba military fort).The picture shows the traditional meeting point at Alhambra gate (el mapa, this is the "map wall")Is it advisable to do the line at counter and try to purchase the tickets on the spot? Absolutely not. If you Book your private guide you will never do lines.