How to be a good student.. Help please

I want to study medicine... and all this time at school, I have never really worked to my full potential. When I was a lot younger and had no distractions like facebook, friends, parties etc and topped my year in everything. Since I entered secondary school, I've just relied on my intelligence and crammed/'winged it' for exams and class tests.
This year, I managed to scrape through my AS's with the required grades for medicine, and I've realized how much I've slacked when I see that the people I was once competing with/doing better than are now in a way better place than me academically.
Reasons for this: I'm lazy, INCREDIBLY disorganised (I've always been this way) and find it impossible to stay on top of my work. I LOVE working under pressure, and I find that I have long periods of time where I just sit around, watch tv, read, tsr and then series of all-nighters trying to catch up with everything.
This year (A2s) I want to put it all behind me and get into good study habits for uni. But I'm finding it so difficult - Keeping organized is a NIGHTMARE first of all, I'm always forgetting my school timetable/what homework we have/what day it is! I have a rep for this kind of thing at school. When I do start working for something, it takes me ages. Writing notes is a nightmare because I can't summarise to save my life and end up writing an extended version of the chapter! It took me 5 hours to revise for a class test yesterday and I still didn't do well!
Sorry for this long nightmare of a msg, but does anyone have any advice on how to be a good student? e.g. how to write notes/keep on top of things/revise perfectly for tests?

Thanks