I have a pond which we've always kept koi and goldfish in. :) If you have any questions, you can ask me and I'll try my best to answer them. :3
I personally prefer koi over goldfish, because a) Koi are very friendly fish, I've taught mine to eat out of my hand. :) b) They come in a much bigger variety of colours and grow larger and c) Goldfish reproduce like rabbits in ponds. We've gotten over 100 babies at one time before. :P
If you're going to get goldfish, I would recommend to get either common goldfish, comet goldfish, or shubunkin. The fancy types don't usually do well in ponds, as they can't compete well with single-tailed fish for food (if there are other varieties in the pond) and it's harder for them to escape predators. Fancy goldfish by themselves would probably be okay with netting.
It's a good idea if you live in a very warm area to have some shade over the pond. This can also help protect from birds, as they won't be able to see your pond/ fish from the air.
And if you live in a cold area where the pond might freeze during the winter, you'll need either a pump or a heater to keep it from freezing.
Here's a good website with lots of information on koi: http://www.nishikigoi-info.com/index.html
Most of it that isn't specifically discussing koi can be applied to goldfish as well.