But how do you stay on top of things? There seems to be so much to do, and learning the lecture slides are a lot harder than I expected- things just won't stick in my head, and it seems to take a lot longer than what I anticipated. And I am tired like ALL the time.
1) It'll come to you, nobody got good at wrote-learning in less than a week. 2) Tired all the time since Monday?

I'd suggest taking a little bit of time just to chill, HSFY only gets more content-dense as you get into the semester, it isn't going to get any easier, so it isn't worth stressing yourself out over week one's stuff. It isn't a sprint-race, it's a marathon, so just go
steadily at a pace you can keep up for an entire year, and you'll be fine.
I'm freaking out about HUBS, the lecturer goes so fast, and I try to copy down all the extra notes, but always seem to miss a considerable amount. But the slides themselves are quite information- heavy, too. For the exams, would we be okay if we learnt what is just on the slides?
Yes, the HUBS lecturers go fast (just wait till repro in HUBS192, where one lecturer goes through 50 minutes worth of content in <20minutes...), but as has been pointed out, the slides will be available on blackboard if you can't keep up. For what you write down during the lecture, if you have to choose between writing down what's on the slides, and writing down the stuff that the lecturer says that isn't on the slides, write what the lecturer is saying (because that's something that it's way more time-consuming to go back and find later, it's a lot easier/quicker to read some slides than listening to an entire podcast).
Yep, it has been said a few times by med students on here who have done HSFY that as long as you know the info on the slides you're ok. There might be 1 or 2 questions on the exam which are based on external material or simply what the lecturer says, but don't worry about that.
Agreed re: textbook readings, there shouldn't be more than one or two marks in an exam based on those. As for 'what the lecturer says', you'd be surprised, especially if the seemingly random stuff they're 'rambling' about happens to be their area of research....