You'll do two years - AS and A2.
Let me tell you, dont mess about at AS and think its only A2 that matters - this isnt the case.
Their is a step up from what is required from you when moving on to A-Levels (you are no longer spoon-fed information like at GCSE).
The step-up from your first year of A-Levels to the second is also evident.
Do good in the first year to lay some sort of foundation for the second year.
I was often told that it was hard to go up more than 1 grade from AS to A2 (for example, you get a B at AS, so if you work REALLY hard, you could get an A)
Also, dont leave things to the last minute - GCSE's were easy for last minute revision, but at A-levels, you have to remember that at the end of them, you'll be competing with others for Uni places.
Obviously, dont sacrifice your social life, but instead be realistic about it. For example, take Friday evenings off to socialise or something.
From my experience, I didnt put enough effort in to A-Levels (always being told I'll 'do well' and being predicted A's all the time didnt help).
But at the end of AS (first year), I came out with A B D grades. I was predicted straight A's.
I know I had to work EXTREMELY hard the next year, and for the first time ever, I was staying up late to revise even more.
I worked especially hard on my 'failing' subject, did a retake on one of the exams (got it from a 'D' to a 'B').
At my school, we had 'free periods' where we could chill in the Common room. I can tell you that I never once spent my free time in the common room at A2, but instead was in the Silent Study room or library.
The hard work did pay off, and I got A A C overall - more than enough for uni.
This isnt to say that its 100% the hardest thing in the world. It isnt.
But you need to look at:
What you want
What you need
Your capabilities
What you do with free time
And more.
If you want to do something highly competitive, then you need to work hard. And from the start.
Dont give yourself and uphill struggle that leaves you worried like crazy on results day.
Good luck, and well done for choosing further studies