If you are hiring a car from a reputable firm such as Avis or Europcar then you won't have anything to worry about. You will be insured on the car but if you want any additional drivers they will need to be added (at an additional cost). The firm will explain if you have any mileage limitations, plus usually they will loan you the car with a certain amount of fuel in and it will need to be returned with the same amount.

Drivers must have a full clean driver’s license for a minimum of 12 months at the time of rental for most firms, and there will be a Vehicle Damage Cover Excess plus Vehicle Theft Cover Excess. If you want to reduce the excess you can purchase additional cover when you pickup your car.