Some cars have a mesh halfway down the pipe leading to the tank to stop people stealing the petrol.
If your's doesn't try this.
As you put the hose pipe in to the tank blow into it and you'll hear bubbles when your'e in the petrol.
To get the petrol out, don't suck as 'dee' sugests, a mouthfull of perol isn't nice. instead place a damp cloth around the filler cap to form a seal around your hose and blow, hard, this will increase the preasure inside the tank and when you stop blowing the petrol will be forced out through the hose. Keep the end of the hose below the height of the tank and the petrol will continue to flow until the tank is empty.