Ford Fiesta Petrol Cap

[fiestapiggy]

Just bought a Ford Fiesta 1.8 LX, and having issues with the petrol cap. It's a 97 Diesel, and whenever I try to take the cap off, it is very difficult. Not sure whether there is a special way to turn it or something, but haven't managed to work out how to do it. There is no manual, and this is my first car, so I'm quite stuck as things are. I was shown how to take off the cap, but it's not so easy on my own!
Any help/suggestions welcomed.