How to start a fish-pond?

I have enjoyed caring for fish in a tank and have a lovely little pond in my backyard that I would love to begin filling with fish. Sadly, I'm not quite sure where to start



For reference, my pond resembles that one. It is a little bigger with a taller two-level waterfall.

Koi are definitely an option, but I was wondering what are some other fish that could do well in a pond? I would like to have some diversity and not have all one type of fish, if possible.


Also, are there merits in making in making a pond have "bi-levels" or a little half tunnel like the gorgeous pond pictured here? Also, are there any must-have plants? This is a bit of a far-off project and I wouldn't start until spring. Just wanted some opinions.

Thanks so much!