Hi Janet, I found this lovely tutorial for you that will show you how to build a small goldfish pond - https://dengarden.com/landscaping/How-To-Make-a-Gold-Fish-Pond Have fun
Here is some information from Pinterest https://www.pinterest.com/pin/644859240370971470/
These should give you some ideas:
I have a koi pond, have fun making your goldfish pond - you won't regret it!
A pond for goldfish or water lilies need be only about 2 feet deep for zones 5 or greater. Ponds built in colder areas may need more depth to keep the pond from freezing solid. Ponds built for koi should be close to three feet or deeper to allow these larger fish enough space.
Pond fish depend on pumps both to filter their excrement from the water and to keep the level of dissolved oxygen in the pond water at a suitable level. Fish may live for a short time in a small pond without a pump, but will quickly run in to problems unless the water is frequently changed.