First of all, welcome to The Break Room! You may want to 86 your location for the sake of anonymity.
Here is what should* happen. When you start your shift, ask for the GSTL, who will get you started. You'll get a cashier training guide to read through. You'll be paired with a peer trainer, who will use a register in training mode to go over the basics of cashiering and demonstrate different types of transactions with you. After you finish on the training register, your trainer will go on a regular register and cash out guests as you watch and learn, then when you're ready, you'll jump on and cash as your trainer observes. If, by some turn of events, your trainer's shift doesn't line up with yours, you'll be paired with a trusted veteran cashier.
*Different stores handle training differently. This is how I was trained when dinosaurs roamed the earth (three years ago). I was seasonal too.
The register software is pretty straight-forward, so you don't have to worry about that. If you're not used to dealing with people, cashiering can serve as a crash course - you get used to it after a little while. This time of year, retail can get overwhelming, but you just have to remember to breathe. If you need help and/or don't feel comfortable with something, talk to your GSTL (or softlines TL once you start training on the floor).