This is a very complex question.
Most companies will provide the vehicle and then (road) tax it, insure it and pay for maintenance.
Some companies will give you a petrol card. Some companies will allow you to have personal as well as company mileage (this is taxable so beware).
Some companies will give you a mileage allowance to take care of the petrol costs (often 40p per mile).
You should be aware that driving to and from your main workplace / office is classed as private not company mileage.
There are a variety of tax issues and it is dependent on the cost price of the car (retail price not discounted price) and the CO2 emissions.
If you have a choice go for a cheaper diesel car. Do not be tempted to get things like Sat Nav as an extra as you will be paying tax on this.
A lot more details here:
http://www.hmrc.gov.uk/cars/