Thank you- this helps a lot.First thing I'd do is fill with water and see if it leaks. The size looks too small for koi, so goldfish are your best bet. You'll have to look for any tykpe of filter and see if it is functionable. After you've verified no leak, you could add some plants, a bit of ammonia, and wait for the nitrogen cycle to finish (30 days or so, I think). This would be a fishless cycle and once completed, you could add such goldfish varieties as comets, shubunkins, fantails, etc. Keep the number small and you won't overload whatever filter system you have.
And once you start, read up here on the various topics you're unsure of; there's plenty of info to go around. We can help with any further questions. You can get an API water testing kit to help understand exactly what water quality you have and are aiming for.
Hope this helps.