Sorry you had such a crap first day.
*hugs* It's an incredibly stressful time of year for everyone involved in the retail experience [guests and team members alike] and seasonals are all over the place. Some may leave willingly before the seasonal bit is up in January, and some may get the boot early for whatever reason, so maybe your team is waiting to see if you stick around?
I was lucky to get a great team. My trainer was super friendly; we chatted almost non-stop while working, and she even managed to cover just about every base there was to softlines. Even showed me how to print labels and answer the phone, as well as write in the log book after checking through the fitting rooms. There are times when other team members are social, and there are times when they would just rather keep to themselves, and I definitely respect that. I've even hammed it up with other cashiers and hardlines folk, and said hi to a couple of fellow team members outside of work when I run into them. Also, while it didn't really affect how a couple of them interact with me, my husband came into work with me one day to do a bit of clothes shopping and I discovered that two of my co-workers are buddies of his from his previous job or high school!
Edited for: Totally forgot to say; keep your chin up! I was assigned Softlines when I started my seasonal bit, but I'm being thrown on the register a lot. It definitely gets easier as you do it more often. I started out kind of withdrawn socially and stumbling over my words, but now I greet every guest with a smile, ask them how their doing, I've gotten 3 redcards today for the first time, and I have so many vibe moments with guests it's unreal. I have at the very least 10 vibe moments a day, ranging from talking about movies to pets to makeup and nail care.
A tip that will perk some customers up; if your coupon machine spits a coupon at you for something they're ringing up and you haven't hit total yet, scan it! Keep an eye on your screen when you do; sometimes it won't say anything at all and take the coupon, sometimes it'll pop up with a notice saying that the coupon can't be used as there aren't any qualifying items [example, had a few today for a certain amount off two purchases of the big Kleenex bundles when guests were only buying one], or it'll just say "Hit accept if the item is in the purchase". I've made quite a few people happy with that.
And as for checking under the cart; most of the time I've found guests will push their cart forward so they can access the card reader, or they are nice and move their cart so I can reach over our tiny gap and place their bags in the cart. That's a good time to keep an eye out for misplaced items. Check the inside of those massive makeup bags too, but mostly just ask the guest if they want the paper kept in so they don't get the impression that you're being nosey and suspicious of them. As for the massive kid carts, offer their young ones stickers! More often than not I'm leaning over the counter to hand them their sticker/s and it's a good time to check the seats. Don't mind the incredibly awkward stare the really young toddlers give you, I don't know what that's all about.