I've just finished my last year of Sixth Form, here is my advice and the things I learnt:
1. Don't over-work yourself during the summer between GCSEs and starting Sixth Form. You won't get a summer as long and as free as this one again for 2 years, so enjoy yourself.
If you want to do some work, good advice would be to (a) complete any induction material your college has sent you and (b) familiarise yourself with the structure of each A level you're taking (modulus etc).
2. Starting A levels: At first, in my experience anyway, they seemed absolutely impossible. But do remember that you're only doing 4 or 5 of them, compared to 10 subjects at GCSE. By the time your exams come you should be a lot more experienced.
3. Revising: I didn't start properly revising until the start of the Easter holidays. A very good tip is to get all your notes made as you go along the year, reviewing them at half terms and in the Christmas holidays. At GCSE my exercise books were in such a mess, so I had to spend the first half of Easter writing up notes. That was valuable revision time wasted.
4. Making Friends: This will seem hard at first, but the usual advice applies: be yourself, don't be afraid to put yourself out there, and keep trying. Its invaluable preparation for university. You don't have to be the most popular person in the year, just aim to have a few friends who you get on well with.
5. University: During your Lower Sixth year, you should be mainly focusing on your exams. But this year is where you should start to think about where you want to go, and which course to go for. When exams are over, start going to open days and visiting places (most universities have open days around late June). You can easily visit a place out of open day, just go to reception and ask for a campus map. You should create your UCAS account (the website through which all applications are handled) after your lower 6th exams. Write your personal statement over the summer holiday then.