Ok, let's be pragmatic for a moment: of course, the choices are up to you, but in the end, your final destination will be the result of two key points - your performance at interview (if you have one), and the results that you get for your A-Levels.

Now back to the beginning. All you know is that there is a UCAS form with space for 6 university/course combinations, but no idea what to put into the spaces.

Uh Oh..

Start panicking because it's good for you to realise the magnitude of these choices and realise how much you have to find out, then stop.

Start at the beginning, with the broadest and most general questions, and once you've figured those out, start thinking a bit deeper.

Choosing a course and university are kind of linked 'cos you can't have one without the other.

You can decide where you want to go to university and then see what courses are available....

But its best to start with a course (or more realistically a subject) you're interested in doing to degree level.

Interest

You must choose a course that interests you. Don't even think about doing a course because your best friend is, or your parents think its a good idea.

For sure, listen to other peoples opinions, especially if they have been around a bit. Teachers, careers advisors and sometimes even your parents might know what they're talking about. But don't immediately believe everything without question. People make mistakes, and you have to do the checking. You're too old to be blaming other people for something you should have worked out yourself.

My best subject?

Do not just choose your best subject either. You will be living this subject for the next 3 years plus, so do it because you want to, and not just because you are good at it.

You may think you are good at it at A-level, but it doesn't mean you will be at degree. Also, if you're not interested, you won't put in the work, and you won't get the grades at the end of the day. Don't think you'll be able to blag it either; yes, people say that A-levels are the hardest exams you will do. Unfortunately, they're lying; it just makes you feel better now.

Sorry.

Continue an A-Level to Degree Level?

Many people continue on an A-Level subject to degree, but it's not necessarily the only choice. Investigate and see what else there is, but it's a good bet that if you've stuck at an A-Level for 2 years, you'll be able to continue it for another three.

Be wary of starting a course you haven't studied before.

Something like Archeology springs to mind. Make sure you know exactly what you're letting yourself in for before you devote the next three years to it. The novelty can soon wear off (remember it's hard to change course, and you'll probably lose a year in the process).

Now that you've got some ideas, continue on to find out about picking a university.
Or, go back to higher education.