a/s/l?
That and how much can you afford on insurance as if you are 17 and just passed then just about anything will be expensive.
When i was buying my first (and current) car my only rule was that it wouldnt be some old angled car, i wanted something that looked like it had been designed recently and not 20 years ago. I ended up with a 4 year old Focus Mk1 and have been very pleased with it.
Cars are banded into insurance groups so say a 2.2 Mondeo is group 13. a Aston DB9 is Group 20 (the highest group). The lower the band the lower the insurance.
http://www.parkers.co.uk/insurance/Groups/by-group.aspx?ig=1#ig=0 is a list of cars in the lowest band. You'll note that you probably don't want to drive any of them. Also note that many types of car are in multiple groups (1.0 Corsa is group 1, 1.2 Corsa is group 2 etc). have a look for cars you like and then see what band they are in, that'll give you a rough guide as to insurance costs without having to fill the poxy things in all the time. Also remember that your postcode plays a part in that cost. Some people tell fibs and say the car is at a different postcode (uncle or something) to save a few quid but should you ever need to claim and they find out you were lying the'll not pay a penny out regardless of whose fault it was. its just not worth it in my view.
I've only ever driven Ford Focus' (foci? focusi? focusies?

) since i learnt to drive in three and then bought one. I have a 1.6 Zetec which is great fun to drive solo and also practical if you are loaded up with peeps and luggage. The wife has a 1.4 Fiesta Diesel for work which gets good mileage and is a nice little car and is only £30 a year to tax compared to my £155
Ultimately a lot of it comes down to how much you can afford to keep it running year on year. Insurance/fuel/servicing/road tax/tyres all pile up and that's before you get to things just stopping working like coils or exhausts or whatever.
As an example i do about 7k miles a year and i get about 30 miles to the gallon. Over a year thats about £1200. The wife does about 12000 miles for work but gets nearer 40 miles to the gallon. That's about £1500 a year. 4 tyres for either of our cars cost anywhere from £170 every couple of years, a full service is about 150 if nothing else needs doing. an mot is about £50 these days and both our insurances are about £400. All in all if nothing fails on my car it costs me around £1800 a year to run.
good luck finding your car, the choice is crazy and you'll get lots of different opinions from people with all kinds if views. My dad would say avoid everything French and everything Japanese. I'd say that i like Ford and spares are cheap as chips compared to say VW.
Whatever you get try and find an owners club that has an online forum, the Focus club (FFOC) i'm in is £20 a year but the help available on the forum has been invaluable when a few bits went wrong and in themselves saved me a fortune by not having to go to a garage.