screen resolution

[Buttocks]

I have i nstalled ubuntu from the free disc in Computer Active magazine. When I changed the screen resolution so that the picture I wanted to display as wallpaper was better, I could not change this back again. I have decided to reload the whole of Linux again as everything is too big on the screen. But if I have this problem again what do I do? I need explanations in very easy to understand language including where to find any file or programme. Thank you
Marion