Knowone has actually stated at what age they had there first car, so everyone saying what they had their insurance price, isn't that helpful in my opinion.
The topic though is a good idea.
I passed just over a year ago when I was 23, my first car was a 1998 R reg Vauxhall Vectra 1.8 petrol, cost me £960 with me as policy older and main driver with my girlfriend as a named driver to bring it down as she has been driving for 4 years now.
The thing I found when insuring my car, was that I was getting cheaper quotes with fully comp compared to third party f&t. I would recommend checking both when getting quotes just incase you can get better cover for less.
I would suggest anyone who is age 17/18 to avoid cars such as corsa's and saxo's, as these are the cars that most boys buy and try and do up, rag and crash. So an insurance company is likely to charge more for a car like that as they will assume that based on your age and the fact your male that your gonna do the same. Maybe try diesel car's and see if they come out any cheaper.
I got rid of my car before christmas last year as it was going to fail it's MOT, I decided that I didn't need a car, as my girlfriend has one, so I am a named driver on her policy, and amazingly I reduced her premium by about £200, yet I have only had insurance for 10 months and so have no no claims earnt. Though when I was looking at cars, I was finding that diesels to be cheaper that petrol.
If petrol is your only option, then your are limited to a 1l, 1.1 and possibly a 1.2. But as it's your first car, it's best to get some old and possibly a bit scratched, as chances are your only going to scratch it some more. I was told by my instructor that when your learning to drive, you are only being taught to pass the test, after that when you can legally drive on your own, that's when you really learn to drive.
As someone mentioned Parkers is a great site to find out what insurance group cars are in.
Hope this helps.
PS: I wasn't bothered about driving till about a year or so ago, hence why I only passed last year.