I would suggest you also post at: Ponds and Aquatic Plants Forum - GardenWeb - "Ponds & Aquatic Plants ... is meant to cover the design, construction and maintenance of ponds, as well as the subject of aquatic plants in general..." They may all click here too, or may not, I am not sure and since these are low traffic forums duplicity is no disadvantage.

They will want a guesstimate of volume of water, depth, is it too be pumped and filtered, and your zone.

And, think about sarassa too.

What above poster said - that space is nowhere near large enough for koi, which get 2-3 feet long. They require about 4-5 feet depth + minimum 1000 gals per fish. Sarassa comets (ie type of goldfish) are better choice and can get very koi-like with long flowing fins - however Im not sure its deep enough for even comets. If not red minnows are great for small ponds.

I would start by clearing out muck and decaying organic debris - if you dont have a compost pile now would be a good time to start one!

First see what you have - how deep, how do the 2 parts connect, are they same level or does one meant to flow into the other? Is the liner still any good?

I wonder if it was designed to be a pond or a water garden?. Identify what the established plants are - looks like there may be some iris? - and which you will want to keep.

Or a bog - bottom photo looks like a bog. Perhaps water flowed through there (if it was intended to be a bog) and then flowed down to the other section? Or perhaps it was neglected so long that the plants started growing in the debris.

In any event I think I would just start by seeing what exactly you have and then posting that, along with your ideas of what you would want it to be, on the pond forum.

Yeah, not sure why this forum is here - no expert koi keepers posting that I can tell. I just stop by occasionally - 9 times out of ten its just letting people know their ponds are too small for koi LOL.

EDITTED TO ADD - first step before cleaning out muck -identify the live established plants bc you may want to be keeping those and maybe all it needs is the leaves cleaned out (somewhat) and the pond to be refilled if its to be a more naturalistic pond.