If you just want normal Alhambra tickets book them at the official site.

This is the official site

https://tickets.alhambra-patronato.es/en/

If you want a guided tour get one here:

https://granadainfo.com/alhambra_ticket_search.htm

If you were too late an there are no tickets left follow the advice on this page 

https://infoaboutspain.com/article/alhambra_tickets_sold_out

DO NOT BUY TICKETS FOR MORE THAN 24 EUROS:

Even if the web site is well made and it looks official it is not. Some sites charge 4 times the real price and then let you know the day before that there is no availability. USE THE OFFICIAL SITE LINK ABOVE.