Welcome to the forum. Do as much research as you can before digging the hole. If you are keeping koi you need at least 4' deep. Get the best pump you can afford the first time, low watts, high volume, external. This wont be cheap but in the end you will be happier. Dont skimp here. Build yourself a good biofilter, skippy or doc type. Dont stock to many fish at first. Especially koi. I would start with a couple comets to get things going in your eco system. Add as many plants to the water, lillies, hyacinth, lettuce. Your want lots of circulation in the pond, this may require a extra pump set in the water. Add some type of bottom drain to keep the bottom clean and pull water from the deepest part. DONT put rocks on the bottom of the pond. It will turn into a cesspool in 6 months to a year and you will be taking them out. Avoid shallow areas where predators can wade into and feed on yor fish. This is just a small start. Good luck, we are here to help.