You just have to stick to legal, regulated online casino sites such as BetMGM, Caesars Palace Online Casino, FanDuel, Betway, bet365 and BetRivers. They offer great casino game ranges and they will pay out quickly and without any issues if you win.
The state does not permit real money online gambling in California, so the only legal option is to visit a social casino site. These California online casinos allow you to play for fun, but some also award you Sweepstakes Coins, which can be redeemed for real money prizes and gift cards.