Here's a short list:
Pisser/Crapper/etc. - Toilet.
Wetbacks - Mexican immigrants, usually illegal (reference to swimming across the Rio Grande). Note: Mexicans themselves will often refer to illegals as "moes," shortened from "mojado."
Beaners - People of Mexican descent, but not necessarily immigrants (I warn you of these because people will think you're Mexican).
Crackers - White people, especially white trash (which is itself a slang term which is to whites what n*gger is to blacks).
Hicks/Rednecks/Hillbillies/etc. - Usually in reference to American southerners, especially rural folk, but can mean any kind of white trash.
Scrubs - The lowest level of any group of persons (similar to 'grunts' in the Army - low-level infantrymen). It was originally used to refer to "strikebearers," or non-union workers that would take the jobs of the union workers when they went on strike. It is mostly prevalent in sports culture these days, and refers primarily to the guys that never get off the bench. It was also popularized by TLC in their song "No Scrubs," referring to men with no "game."
Game - The ability to woo a woman.
Birds - Women.
Pigeons - Really nasty women that no one wants (in reference to the fact that a pigeon is the lowest form of bird).
Skank/Hoe/Tramp - Any kind of dirty/nasty women as well, although it is generally specialized to sluts, which are women that are really "easy" (to get into bed, that is).
cool person - Asian person of Chinese descent.
Gook - Asian person of southeastern descent (especially Vietnam).
Boner/Hard-on/Woody - An erection.
Morning Wood - The erection that men often get in the morning.
Minuteman - A man that doesn't last long in sex.
Goth - People that usually dress in all black, wear pale makeup and dark eyeliner, listen to The Cure, Siouxsie and the Banshees, etc.
Emo - Dumb teenage kid that is sad all the time and listens to emo music (which is somewhat hard to define, but is characterized by despair over breaking up with women and constantly being sad).
Greaser - People that like to dress in the 50's outlaw style a la the film musical Grease.
Swingers - Can refer either to people that enjoy swing music/dancing or people that engage in wife-swapping and orgy parties, etc.
Mallrats - People that hang out at malls for no particular reason.
Boss/Cool/Sweet/Killer/Tight/etc. - An emphatic way of expressing approval for something. Each has a more specialized usage that you don't really need to know - you'll learn the nuances eventually.
Down-low - Secretive, quiet. Note: This is not used as an adjective, but as an imperative (i.e. "Keep it on the down low").
Tweaker - A person that uses speed (methamphetamine).
Stoner - A person that uses weed/reefer/pot, all slang terms for marijuana.
Alky - A person that drinks compulsively (shortened from "alcoholic").
Lush - A person that drinks excessively, but only as a social drinker.
Fairy/Queer - A homosexual male.
Friend of Dorothy - Archaic and infrequently used, but also a reference to a homosexual male. Derived from Dorothy, the character played by Judy Garland in The Wizard of Oz, as gay men stereotypically are fans of her music.
Queen - The man who plays the role of a female in a male homosexual relationship.
Butch - The man who plays the role of a man in a male homosexual relationship.
(Note: These roles are not inflexible and do not represent general rules, but rather stereotypes.)
Dyke - A homosexual female.
Bull-dyke - A particularly manly homosexual female.
The 818 and The 909 - The San Fernando Valley, and Riverside County, respectively, in referennce to the area codes in those regions. Those are the only two areas I can think of that are referred extensively by their area code. "The 909" in particular is often seen as a derogatory, as the inhabitants of Riverside County are the rednecks of California, looked down upon by the rest of the state. To a lesser extent, the inhabitants of Northern and Central California (especially the rural areas) are looked down upon by Southern Californians. The monikers SoCal, CenCal, and NorCal are used to refer to these regions in shorthand.
Bling, Bling - Any piece of extravagant jewelry, or just riches in general. Note: "Bling, blingin'" is also used as a verb, to mean that a person is openly flaunting riches of some sort, or just living the high life and loving it.
Robbing the Cradle - The practice of dating, and especially having sex with, something much younger than oneself.
Jailbait - A girl under the age of 18 (the legal age of consent) that is extremely attractive and makes you want to commit statutory rape.
187 - Murder, as in "187 on a muthaf*ckin' cop." From the LAPD code for a murder. 212, used less frequently as slang, refers to a robbery. 212 is also sometimes used to refer to Manhattan, as that is the area code there.
Freaking - A kind of dance where the male humps the female rhythmically from behind. This term is also used to refer to men that come on to women very strongly (although this usage is becoming very rare) and to dry-humping in general.
Grooving, Popping, Breaking - All various forms of urban dance.
Kick-Back - In general, this simply means to relax, but high-school and college age kids in California use this term to refer to specific kind of party that is usually held during the day, that doesn't necessarily involve loud music or dancing. Often times, people just get together to drink and "shoot the sh*t" (which means to engage in small talk).
Off the hook - An adjective meaning that something is very happening. "That party was off the hook. That girl is off the hook."
Off the heezy - Also means off the hook.
Off the hizzle - Also means off the hook.
Off the hizzle fo shizzle - Off the hook for sure.
Fo shizzle ma nizzle - For sure, my n*gga.
("Off the heezy fo sheezy" and "fo sheezy, ma neezy" are also used.)
Kicks - Shoes.
Ride - A car.
Lid - A hat.
Baller - Has several uses, the most prominent of which is a basketball player. It is also frequently used to refer to a man that gets a lot of women very easily.
Uncle - "I quit" or "I give up."
Ghetto fabulous - Something that is overblown and tacky in style, in a parody of inner-city black women that will wear very long nails or hair weaves or extravagant gold jewelry pieces that just look ridiculous.
Gnarly/Tubular - Variants of "sweet," "radical," etc, that are used specifically in surfer culture. Both terms refer to wave characteristics (large waves are often tubular in shape and can result in rather gnarly accidents). Neither is widely used any more.
Bush/Muff/Beaver - Specifically, female pubic hair, but more generally, female genitalia.
Anyway, I'm sure I can come up with more later. One of the big advantages you'll have is that, since southern California is the center of the American film/television/musics industries, the slang from that region is used everywhere in the country.
.
