BTCV have a great book on ponds which you can read online.
http://handbooks.btcv.org.uk/handbooks/content/chapter/386
Bear in mind, this is not about woodland ponds, which tend to be darker places!
Things to consider:
1. Are you allowed to put in a pond? I am not sure, you may need permission. I know that ponds are promoted as good things....
2. Insurance, I heard a rumour about woodland insurance and ponds, someone else might be able to shed light on this one
3. I would also keep it shallow, or at least have a shallow end - don't want any hedgehogs or other creatures drowning in it!
Mike and I have a muddy puddle. No longer called a pond! We found a spring, which was very exciting and dug out some mud, and put in some logs to make a mini dam. The boar think it is amazing and it is no longer very neat! We have a lot of green stuff growing on it now too which we like a lot. I love it, although others may wonder why ;-)