Try driving it 'gently', from cold the mpg will be high initially, say 10-20mpg. After a mile or so it should be up around 20-25mpg. Once warm around town you should get around 25mpg or more. Motorway 70-80mph you should get around 35mpg. If it's a lot worse that this with 'sensible' driving then there is a problem and you need to get it checked.
If you go for a 'spirited' drive you should still get 20+mpg on a standard car.