Hi there, you can pretty much take anything you like and use it to narrow down the list of universities you want to apply to:
-Location. Maybe you fancy York, maybe you don't. Maybe you think they speak funny in Newcastle and want to avoid it (sorry Newcastle - you're a very nice place but Geordie is the stuff of nightmares in the south). Maybe you want to be relatively close to your family, or maybe you would like to be as far away as possible.
-Do they offer your course or not? Not a single university offers every single degree. If you want to study something like computer science, this only narrows down the list a little, but if you want to study, say, diagnostic radiography, it narrows the list quite a lot
-Reputation and rankings. While I strongly advice against basing your decision solely on these things, they are worth considering. I mean, you can go anywhere and still do whatever you want in life, but for the most part, people prefer a place that occupies a respectable position in the rankings. This doesn't mean the top-top universities, but not the lowest ranked. Up to you to decide if you're ok with low rankings or medium or high
-Entry requirements. You'll find that you'll need A*AA for pretty much any course at Oxford and Cambridge but you'll get away with a BBC somewhere else. You should analyse and think what you think you'll achieve in your A-Levels and apply accordingly- apply to 2 or 3 universities with entry requirements you expect to meet, one "aspirational" and one with lower entry requirements. Keep in mind though, always apply to places that will accept you with your predicted grades.
-Fun activities. Maybe you like partying, in which case there are places which are famous for and places that are famous for their lack of parties. Look at the clubs universities have to offer - not every university has a Doctor Who club for example (sad, I know). Some universities will have more fun things than others and some will be more tailored to you (you should research this stuff in the student's union website of each university)
Once you use this criteria and anything else you can think of to narrow down universities, go to open days! Great ways to get a feel for the universities and see if you like them or not.
A great way to find out things about universities are having a general look at their website, it'll have more or less everything they want you to know. For everything else... you've come to the right place. The student room is a great place to read and ask about people's experiences of universities and other things going on with them and it greatly informed my choice of universities both times I applied to university. It can be a daunting at first, but don't worry too much about it, you don't have to decide tomorrow where you want to go, and it's perfectly fine to not have a dream uni. In fact, probably wise not to have a dream uni because what if you don't get in?
Best to have just a bunch of universities you like, some more than others, but that you'd be OK with wherever you end up in from that bunch.
So what you need to do is... don't panic! Truly, don't worry, you just start thinking about a few places you might like, research them online, read up about them on the student room and just do this for a couple of months and you'll start having an idea of where you want to go.