I'm glad that you were able to get decent training! For most of us, we just kinda got thrown to the sharks not knowing what to expect. It's kinda hard to give advice without knowing your specific questions, but I'll try to give some general advice from what I experience when I first started out.
The most important thing is to not let yourself get overwhelmed. There will be times when you are helping Service Desk with a guest, while the phone is ringing, while 2 cashiers need change, while one needs a price check, while the lines are getting a bit too long and you need to call for backup, (Except there's nobody on the floor, so you can't call for backup) on top of the LOD hounding your for Red Cards. So it can get a bit stressful. Try not to do a million things at once, which is the initial reaction I have found to being confronted by these million things at once, and instead take it one at a time. You're not a superhero, and you're not omnipresent. Figure out which tasks are a priority and get them done first. I found that after doing this for a couple of weeks, I had a mental checklist at pretty much all times with what needed to be done and how important it was.
Also, depending on your store volume and coverage, you might need to get used to skipping your 15's and taking your 30/45 at the very last minute. The only time that I ever got to take my 15's were when there was an overlapping GSA shift, which usually meant I only got to take my first and not my last or something. You might need to coordinate with the LOD about who will cover your meal, which should usually be a TL from another department or a cashier that knows the place well enough to hold down the fort while you are gone.
One last thing, don't let the guests get to you. It takes a bit of getting used to when you become "the manager" and any issues are directed at you. This might mean a lot of angry guest interactions. It's important that you develop a thick skin and a strong backbone, otherwise it'll be tough to survive in this kind of environment. I quickly built up a reputation for being the "takes no shit" or the "stoic" GSA since I generally don't let emotion show on my face and I have a fairly thick skin.
There are a lot of things about being a GSA that we can't prepare you for as it's all learned through experience, kinda like with the Service Desk. There is just no way to adequately prepare you for what you're gonna face. A lot of it will make you question whether you are in a dream or not because it'll make no sense whatsoever. You might also find yourself contemplating homicide.
