Super stressed. Asking what I can do to make things better.

I know people constantly complain on here about working for Petco but I personally am just grateful for having a job. It’s tough out there right now and I had a hard time finding a job let alone one that lets me work with animals and not just regular retail. At least I get to pet dogs everyday. My only issue is that since I started 3 weeks ago, I am constantly at the register which I don’t mind. I didn’t mind it until yesterday when I was asked about getting fish by a coworker and I said I didn’t know how. He really dug into me about it and insisted I should have asked to be shown way sooner and I said I did bring up how I didn’t know how to properly get animals/fish for guests and was shown one time how to get mice but I can’t learn all the steps with the zebra and everything with only being shown one time and they never made an effort to take me along and show me any other times. He then told me I need to pester whoever is the leader on shift about it. I just don’t want to be annoying them about it. I figured if they wanted me to know how to do things they would train me on it. I also can’t get away from the register and take it upon myself to watch someone do it, when I do leave even for a second and a guest comes up and has to wait I have been lectured on making sure I’m staying there in the general area. How am I supposed to follow someone over to fish/ other animals and watch them get the animals and actively learn? let alone do the BOPUS orders if I’m always stranded at the register? I don’t mind doing other tasks even if I think I should be getting paid more if I’m doing every area of the job tbh because I only get paid $12 but like I said I’m thankful for having a job and so I would do it if I’m required to. I’m just sick of going to work and being worried I’m going to be yelled at for not being trained for things when the manager should be yelled at for that and not me but that’s just my opinion. My manager also told all of us I’m going to be the only new hire even though he’s technically allowed 2 more people on payroll which stresses me out even more because I’m working almost full time hours (I specifically interviewed for part time due to chronic pain and needing to work less than average) and was told I would only be working 15-20 hrs a week before I agreed to the job and clearly they don’t have enough people for each department or it wouldn’t be a problem if I don’t know how to do everything in the store. I just need some advice on how to best approach all of this and some encouragement that I can do this because I need the job badly and I really do like parts of it, it’s better than other retail jobs I’ve had and I really want to make this work.