I'm not familiar with CA gun law or what the HSC is. I took an NRA full-day handgun safety course, which I highly recommend. I also got a Concealed Carry Permit for my state (if your thinking about this - then add a holster to your list). In addition, I had a 1 hour 1-on-1 instruction with one of the instructors at my local indoor range -- not necessary, but it was very helpful. Lastly, I recently purchased a book on tactical shooting fundamentals... can't recall the author or title -- but the book is a great teacher and reference as well. Also, FWIW -- I made a point to handle my new gun at least once per day when I got it, just to create and reinforce safety habits of immediately checking the magazine, checking / clearing the chamber, etc... you absolutely cannot get complacent with, or take for granted, a strict and solid safety regimine. Start building good safety habits right away!
You will definitely need hearing and eye protection. Because these are personal items and they're not terribly expensive, I like having my own. There are also many options in terms of decible reduction and glass wear, so you may have particular preferences or needs. On the other hand, if your local range provides these items, you're comfortable with them, and they're free -- then maybe you don't need to buy any of them.
Cleaning kit. I've found there are almost as many cleaning kits and methods as there are gun owners. And the Glock manual is not very specific. I called Glock directly and they told me to use Breakfree CLP, a bronze barrel brush, patches, and a bronz toothbrush scrubber (you'll need a rod for the barrel brush and patches). That's all I use. There are lots of Youtube videos demonstrating field stripping and cleaning.
As far as lockbox. First decide why you want a lockbox / safe... lock guns away from kids, keep from being stolen, how accessible do you want the gun to be, etc.... Then start looking around. There are lots of options and costs. If kids are a factor -- then I would get one BEFORE you bring the gun into the house.
Ammunition. Do some research here and elsewhere on practice ammo and defensive ammo. Early on I was sold on MagSafe for defensive ammo, now I'm thinking of switching to hollow points. As far as practice ammo, I haven't been particular -- my local range basically dictates practice ammo since I have to purchase it there anyway. Besides that, I get Federal stuff at WalMart for $10 a box. My Glock has never had a problem with any type of ammo I've used thus far.