Powerful enough, but not too powerful = 9mm or .40
Even in Southern Caliyou should have ranges and gun stores in your area which can provide assistance.
My advice is to go to a gun store which also has an indoor range and sign up for a basic course where you can use both a .40 and a 9. That should run about $60 to $100. Afterward, tell them you want to spend $500 on your first handgun and see what they recommend. If there is a Cabela's, Gander Mountain or Sportsmen's Warehouse in your area they will happily take time to explain various options within your price rangefrom their inventories.
For that money, my list includes, Bersa, S&W, Taurus, RugerandCZ as brands which are reputable and enjoy a good following. Remember to invest in more training, a comfortable holsteranda supply of ammunition.
Regardless of what WE think though, you should heft them, sight with them and have the clerks explain operation, magazines and cleaning to you. You have to be comfortable with the choice, we cannot make it for you.
Buying online won't help much since you'll have to pay transfer fees to process the sale through a dealer in your state (probably in your town or city). The dealer you process through will add their fee for running your background check and filing paperwork.
Best of luck and welcome to shooting!