When you pick up your rental car, it will have a full tank of fuel. You'll need to also return the vehicle with a full tank of fuel, or you will be charged for the cost of re-fuelling. This is generally charged at a more expensive rate than the local petrol stations.
It is a good idea to find a petrol station that is close to your return location so that you can ensure the tank is completely full when you return it. If you are returning to an airport location, you'll find petrol stations are often located nearby.
Some car rental companies offer the option for customers to pre-purchase the tank of fuel so they do not have to fill it before returning the vehicle. Note that you will not be refunded for the cost of any fuel that you did not use.
Make sure you know whether your car takes unleaded or diesel fuel, as often the costs incurred from using the wrong fuel is not included in roadside assistance policies.