It's really hard to judge scale on these kinds of things, but I can tell you it doesn't look large enough for koi. They need 500+ gal ponds and lots of swimming room. Three or more feet deep at the least. But it looks great for goldfish if they want some color in there. If they don't care about seeing the fish and just want it for the water noise, then natives will do just fine. Mosquitofish or killifish are my favorites for a pond like this.
To get it whipped into shape, you have a LOT of work ahead of you. Getting out the leaves (and keeping them out come fall) is your biggest priority. You'll need a large net on a long handle and a willingness to be muddy and tired. Scoop the leaves out until you can see the bottom. This alone will take a couple of days if it's as nasty as you say. If you have one, a shop vac would be helpful, but it will also drain the pond.
Next is going to be getting the filtration system in check. Find out what kind you have, and see if it needs to be cleaned. (It most likely does.) You'll probably want to turn it off and clean it. The filter in ponds is mostly for water movement, so you don't have to worry about the beneficial bacteria as much as you do with aquariums.
Finally, you'll want to start looking at plants. Floating plants like lilies and water lettuce are best. Water hyacinth is my favorite, but it's illegal in NC. Floating plants will help take nutrients from the water as well as prevent algae from growing deeper in the pond. You might also want to look at marginal pond plants like iris. Finding a local nursery that sells these plants is a good idea.
But the good news is that this is the start of pond season, and you have the whole summer ahead of you to get this pond into shape. I just spent the entire day working out in my pond trying to get it ready for the summer. It's a lot of work, but so worth it at the end of the day. I'm actually just north of NC. I can see the border from my house lol! If you need any tips specific to this part of the country, feel free to ask! I love ponds, and I love helping people enjoy them. No two are alike!!