Buongiorno, Francesco!
You are going to surfing heaven in SoCal. Be sure to surf at my favorite location there. It is San Onofre. It is often called, San O.
On with the answer you have requested. Go to TK's Frog House Surf Shop in Newport Beach, California (
https://www.google.co.uk/maps/place/Frog+House/@33.6295861,-117.9580774,15.39z/data=!4m5!3m4!1s0x80dd2041245b704f:0xabb7a3ec4ad70fc1!8m2!3d33.6289463!4d-117.9532095). It's stuffed full of new and used boards. Besides, then you are only 3 miles south of Newport Beach. You must go there because it is "Surf City, USA." It's not far from the SoCAL area you want to go to. In Surf City you can surf at the Huntington Beach pier (you'll recognize it from magazines) (
http://www.stockteam.com/hbpier.html); go to the small International Surfing Museum (
http://www.surfingmuseum.org/); go two giant surf shops (one is Jack's that has been there since 1957); eat a great breakfast at the surfers' landmark Sugar Shack Cafe (
http://www.stockteam.com/sugar_shack.html); and have a fine diner, cocktails, or Hula Pie at Duke's restaurant name after the great Hawaiian, Duke Kahanamoku (
http://www.dukeshuntington.com/). All these things in Huntignton Beach are within walking distance of one another. You may also find some good used boards at Jack's (
http://www.jackssurfboards.com/locations/locations.html).
You are going to have a great time!
Ciao,
Billie