Either bus 3 or 33 from "Estacion de Autobuses" to Gran Via stop 1 which is for Plaza Nueva and Cathedral. Fare 1.20 and takes about 30 minutes. See:
transportesrober.com/informacion/lineas.html# for bus route maps.
If you have already your reservation but no ticket you could go to the Alhambra shop in the 'down town' street that leads up from Gran Via to Plaza Nueva ....it is on the right side. Here you can collect tickets using your CC and a free map. You can walk up to the Alhambra with tickets using the street Cuesta de Gomérez and enter through the magnificent main entrance called Justice Gate which is on your left about 15 minutes slow walk once you pass through the Puerta de las Granadas.
Once through the Justice gate you have the fortress or Alcazaba on your left side and the entry for Nasrid Palaces ahead.
See the official web site for more info:
http://www.alhambra-patronato.es