As someone who has just been though a dissertation I'll agree with Peter and say the 100% best thing to do is seek out your supervisor. Typically you'll end up doing a project that is close to their field of expertise and often they'll have something in mind for you, again normally branching off from their own research or the research of their PhD's.
The other piece of advice for you is to try and choose a project that genuinely interests you, its hard to keep the focus required to properly research something if you don't personally find it interesting!