Your common sense is your best tool. Dont worry too much about actual weapons, especially as a foreigner it can only get you in trouble (though you're a woman so it's more understandable).
California, in case you hadn't done your geography, is a very long state (San Francisco to San Diego is about 8 hours drive) so you might want to have an idea WHERE in California you want to go.
Humboldt county, in case you're thinking about the Cannabis industry, can be OK but can also be very sketchy at times. Please don't be naive, we don't want another Netflix special. Arcata & Eureka are fun though.