Hey there! I recently got into this myself. Best place I've found so far is Gaviota Beach. There you can find everything from petrified wood to agates and even fossilized whale bone! You can also try Rincon Beach or Refugio. I've even had some luck finding petrified wood and jasper on the Santa Ynez trail and seven falls trail. I also recommend getting a copy of this book and checking out any spots you find nearby. Curious to know if anyone else out there has any suggestions. Have fun!
I would recommend looking into local rockhounding groups for your area. Here's is a book on California rockhounding. Here is also a list of musems and geological sites to visit in Cali. I'm not American but this is how my parents helped me when they found i was into the subject and now i'm doing it at University! :)