The cheapest way in the short-term is to have your parents put you on their insurance as a 'named driver'. However, this will not do you any favours in the long run, as you will not accrue any no-claims benefits in your name, and then when you want to take out your own insurance in a couple of years without any no-claims bonuses, it will be just as expensive as it would be if you did it now.
The best thing to do is bite the bullet and pay out for your own seperate insurance. It will be more expensive for the first couple of years, but then you will see the benefits of getting your own no-claims bonuses in your own name, and your insurance will become cheaper.
Do you really need a car right now or can you wait a couple of years? You will find that insurance is much cheaper when you are a couple of years older.
Having a cheaper economy small car will be cheaper to insure, stay away from flashy boy-racer cars, stay away from any 'modifications' like spoilers, etc, stay below 1200 cc engine size, keep it garaged, even the colour will make a difference (red cars are more expensive to insure than a green or blue car).
Passing any 'advanced' or 'driver-plus' test, will also help you get cheaper insurance. Again, you have to invest the price of more lessons & test, but it will pay off in the long run.
Shop around for insurance quotes. Get quotes from at least 6 different insurance companies. You will be surprised at the difference in price between different insurance companies.