The first thing you need to decide is your budget.
How much do you want to pay for the car? Don't spend a lot on your first car, £1000-£1500 can get you a very respectable first car.
How much are you willing to shell out on insurance? Your first years insurance is usually very expensive. It all depends on your post code, the car you're trying to insure and other variables. Try and pay your insurance out right rather than monthly. When you pay monthly you're in fact taking out a loan with a high APR rate. You'll usually save a couple of hundred quid paying it off all at once.
Next I would go to an insurance comparison website to get some quotes. Keep all other variables the same but change the car each time. Whichever cars returns the cheapest insurance quotes can make your short list. You can then pick the one you prefer. Try the following cars to start off with:
Ford Fiesta 1.25
Ford Focus 1.6
Ford KA 1.3
Mondeo 1.8
Seat Ibiza 1.2
Skoda Fabia
Peugeot 306 D
VW Polo
Renault Clio
Nissan Micra
To keep insurance quotes down try adding your parents as NAMED drivers (
Don't put your parent(s) as the main driver of your car when in fact you are, this is known as fronting and your insurer can refuse to shell out IF they find out.) Also add any experienced
ladeee drivers in your family as named drivers

.
The trick with insurance is to think outside the box. Most young drivers go for the same car, Vauxhall Corsa for example. Insurance companies do this:
Young male + Vauxhall corsa = History shows young drivers in Corsa's claim a lot, higher insurance premium
Young male + Skoda Fabia = History shows young drivers in Fabia's don't make as many claims, lower insurance premium
The above isn't gospel, but it usually works. A larger engined car does not necessarily mean more expensive insurance

.