Most of the time, everything is computer based, so if you forget, you can always check what a medication's generic or brand name is and what the medication does on your computer screen, but it is more convenient to have those memorized. I'd say just have the top 200 drugs generic and brand names memorized as well as their purposes and the common vaccinations (at what age do you get them, what are they for, how many months apart do you take the doses if its a multidose vaccine, and so on). You should also memorize what should be on a prescription (especially what's require for a class II narcotic drug prescription), whether you can take a prescription over the phone or not, depending on the drug class, and the red flags for when a prescription may be forged. Otherwise, all the systems at different drug store brands are different, so that's something you'll always have to learn again. Your pharmacist should be a lot of help to you to remind you about medication usage/laws, but laws always change every year as does people's insurances, so those are kind of learn as you go.
I worked as a pharmacy tech in my last job, and in California, to get your license, I believe you either need 240 hours of work in a pharmacy or to go to a certified pharm tech school. The one thing I might say is that you don't really need to study to get a job as a pharmacy clerk, or at least, I didn't. Right now, a lot of places are understaffed, and they do need people. I got a job at CVS through a friend, and even if I know you're technically not supposed to do this kind of thing, I would suggest just going to any busy drugstore and asking to talk to the manager to give them your resume and let them know you're interested. I didn't have any experience, but I got hired just like that. Then, you can make money while working towards your license, which is always nice.
If you want a slower store (because some CVS's are very busy and it's difficult to learn), try the CVS target stores. Otherwise, keep in mind that you'll be on your feet all day, and it can get a bit stressful, but don't let it get to you!
Good luck, hope everything goes well!