How many litres of fuel per 100 kilometers travelled does your car use? If you do not know the answer to this question no one can help you as it is your car and not anyone else's and fuel consumption varies widely with driving style, road conditions, state of repair of the car, quality of fuel, tyre pressures, etc. There are many variables.
If indeed you do know the answer to this question, you can work out how many litres of fuel you will need to make a 834 km return trip. The equation will be very similar to this: (8.34 x (litres per 100 kms) x €1.09) = €X, where X equals the number of euro you will need to spend on fuel to make the proposed trip.
The answer to "know how much it takes to fill the tank" can be established by referring to the very useful book that came with your peugeot 206 car under a heading of "capacities" or "fuel tank size".
I hope my answer is clear enough. Mommah always said my answers and explanations were clear, just like the cellophane wrapping on a box of chocolates.