Scared of a long flight

Hi,

In december i have a flight from Europe to Argentina. The plane is an airbus industrie A330-200 (if that matters). It's approximately a 13 hour flight. The problem is that I'm really scared of flying.. I heard that there is a lot of turbulence during the winter crossing the north atlantic. Could anyone tell me something about it? Is it more heavy then during the summer?

Please, be completely honest with me. I want to know what I'm up against!

Thanks!