The most important thing is to chose a topic you are interested in. You'll spend a lot of time working on it, so you need to be able to be enthusiastic about it. I'd also recommend choosing an area that has a reasonable amount of literature, otherwise you'll struggle for sources.

In addition to Jacqueline advice I would say one of the mistakes people make is being over ambitious. Trying at the start to break entirely new ground or do something no one else has ever thought of. Be realistic and be somewhat limited in your brief to keep it manageable and achievable