What I learned at Alevel was definitely different from what I'm learning now. Obviously they go through the basics with you etc, but still it's very different.
As to how the lectures are, it will vary for every university, but it's mostly the same. there will be at least 50 of you per lecture, you'll probably use a lecture room/theatre, the lecturer will stand at the front and he'll use a microphone or shout really loud. you want to do your best and avoid sitting at the back as that's where the loafers and people who don't really want to learn stay to chat and stuff. What most lectures do is teach through the lecture and then you can go up to them ask questions at the end. Some will leave spaces for questions during the lecture but it's more often the case that questions are saved for the end. that way at least if you're asking what you feel is a stupid question, the whole class doesn't have to know
Whether it's easy to concentrate in lectures is a more personal thing, if you are not easily distracted or if you like the topic then obviously you'll concentrate more. It varies for every individual but it shouldn't really be a problem.
You can do whatever you like in lectures, remember you're at uni, no one's going to stop you, you're an adult. So eat, chat, type into your laptop, whatever you want, just try not to disrupt the lecture
As to discussions, you'll find that the department will arrange seminars where students are in smaller groups so they can have more intense discussions and ask any questions they need. This will normally be no more than 30 students and is more like a discussion/debate than anything else. These are helpful so try to attend.
If you do have any questions, remember the lecturers are there to help you so you can book appointments, go to them after lectures, go to seminars and workshops, join/form a study group, ask your personal tutor or ask around and if in doubt, ask us here and we'll be willing to help.
Your last question about concentrating on one thing at a time,i don't know how to answer that because i know all through my schooling life, in every subject, you concentrate on one topic before going to another one. so there's no difference there i don't think, or maybe i've misunderstood your question?
I hope my answers help, if you need more info you can ask here or feel free to pm me.