First decide what sort of degree you might like to do - it might help you to take a psychometric test (there's one on the Total Jobs website called Knowing You which i found quite accurate. Its in the graduate zone, but you might find it useful), but the absolute most important thing is to choose a course you find interesting. Do you have a particular career in mind? If so, find out what paths (courses, work experience) people have taken to get into those jobs. Your careers service in your school or college will be able to help with that.
When you've decided what you want to do, have a look at the UCAS website to find out which universities offer your subject. You can search by area, so you can get a good idea of what is on offer within the distance you're prepared to move away from home.
Next work out which are the best universities for your course. Have a look at the Guardian University Guide as this has course rankings and reports((http://education.guardian.co.uk/universityguide/0,,488282,00.html).
Slmjacs was spot on when she said that different unis are good for different subjects. There are universities that are excellent all round (the top ones are usually cited as Oxford, Cambridge, Durham, Bristol, Warwick...and more. There is something called the Russell Group which consists of supposedly the best unis in the country) but there's no point going to Oxford if your passion is film studies (like me!).
And onto Combined bachelors degrees.... This is an undergraduate degree which allows you to specialise in two subjects. For example, I loved English at A-Level, but was a bit worried about making such a commitment to one subject. I looked around and considered all the combined courses. I read up on each of them, on the internet and in every prospectus i could lay my hands on (you can order these from the unis but your school probably has most of them). Eventually I became enthralled by the idea of Film Studies and narrowed my applications to those universities that offered English Literature and Film Studies. I also took care to find out if I could take different modules from other disciplines on these degree courses. Most universities offer this. Obviously you can't take a Marine Biology module if you study English, but you might be able to take: French, Philosophy, Film, Theatre, History of Art and Design (I took an HOAD module, which made a nice change!) etc etc.
It can be a really really long decision process if you don't know what course you want to do when you start out. My path went like this (this is simplified but gives you an idea):
1) Do I study English, History or Politics?
2) Ok, I like English best - what shall i combine it with?
3) Film Studies is my favourite - which universities offer that with four hours of my house?
3b) Which universities can I actually get into with my predicted grades?!
4) Visit universities - which do i want to apply to? (UWE: the lecturer didn't turn up, Kent: lecture theatres really old (this was in 2001, don't know about now!!)... its really silly but you have to choose somehow)
5) I've got into five places - which do I want to go to the most? I didn't choose the 'best' uni in the end. I compared campuses, towns, travel, halls, courses and my gut feeling. In the end I chose Southampton over Warwick because I liked the modules on offer more. I had the most amazing lecturers and the time of my life.
This has turned into a very long answer, but lastly:
Do a course you WANT to do. Three years is a long time and three grand a year is a lot of money.
Get some work experience in your long summer hols. If you don't know what it is that you want to do when you leave uni, at least do some office temping. You would not BELIEVE how long it took me to get a simple office job after uni (when i still didn't know what I wanted to do!!) because I only had retail experience. It would seem that three years of essay writing, time management and presentation giving counts for nothing with Office Angels!!
Research, visit, ask questions and don't let anyone influence your decision. Its YOUR life and you're the one who has to write the essays, read the textbooks and live in those dubious looking halls (by the way: the more grotty looking the halls, the more fun you have (usually!))