Welcome!
Normally my advice is to attend
Los Angeles County Underwater Unit's Rock, Rips, and Reefs series. They cover the county's most popular dive sites and do skin gear orientations (aka freediving), usually there is an unofficial scuba dive afterwards, conditions permitting. They also have a course called the Advanced Diver program, which teaches divers everything they need to know about diving in the Southern California bite as a recreational diver. It's an exceptional course, and a tremendous value. Sadly both are not happening this year due to COVID-19 but hopefully next year. I always recommend Hollywood divers if you are considering a more technical diving path.
That being said some popular dive spots are White's Point which features warm geothermal vents, Point Dume which has some great underwater pinnacles, the Palos Verdes peninsula a bit of a hike but great dive site, and then you have the Redondo peir (vet's park) with easy access to very deep canyon that brings in a lot of diversified marine life. Perhaps one of the greatest treasures is the Avalon dive park on Catalina island. Great dive park with some submerged structures, great kelp, and usually lots of marine life including giant sea bass, all in an area that is relatively sheltered. Further south in Orange county you have some wonderful easy shore dives from Shaw's Cove and Crescent Bay.
For boats you are in luck, Long beach area holds most of the region's dive boat fleet. I used the Magician, Pacific Star (Pac Star), and Cee Ray. I know a lot of people also really enjoy the Sundiver fleet. Except for the sundiver fleet most are capable of doing liveaboard style diving but many aren't due to COVID. Also our liveaboards are not the tropical private suite style liveaboard but a small bunk below decks type of liveaboard, just as a forewarning. A bit further north you have the majority of the rest of the southern california dive boat fleet out of Ventura and Santa Barbara. Out of there I have dove off the Peace and Specter and both are excellent boats and crews. If you head south to San Diego we have the excellent day boats run by Waterhorse and Marissa, these however are strickly day trip only boats. The Horizon (another great boat and crew) is San Diego's sole (i think?) live aboard style dive boat.
Your 7mm should serve you fine in all but the coldest of conditions, you will want gloves however. Drysuits are common here but not required. Occasionally DUI (the company not the crime) will have parking lot sales at their HQ in San Diego with phenomenal deals so keep an eye on their facebook page.
