If there are no other problems, then it's all down to your right foot.
Otherwise, check the tyres, which make a huge difference. Should be 37psi or 2.6bar as on the inside of your fuel filler flap.
Take it out on to the motorway and stay at a constant 60mph in 6th gear (no heavy right foot, no overtaking) for about half an hour and see what the average mileage is - should be a lot higher (high 30s, low 40s at the lowest). If not, take it to the garage for inspection.
When I bought my car at 3000 miles, I was managing 22-25 mpg normal city driving.