I'm not sure how long tickets will be available as I didn't follow sale figures. You may check it regularly at ticketmaster.es.
You have to collect the tickets in person, from the moment you bought them till the start of the match (and probably even after that). You have to show your card used to buy the tickets (and probably your passport/ID) at a ticket office:
http://www.fcbarcelona.com/info-tick...hasing-tickets You may try if it is available for print at home at the official site (Tickets section, Tickets pending printing link) but I heard it rarely works for some reason.
The match will be on 23rd of August, 22:30 CET (GMT+2, local time), so it is 20:30 GMT, see here:
http://www.fcbarcelona.com/football/...na_real-madrid