You can buy black plastic shaped ponds or buy rubber pool /pond liners you can make in my shape you dig out. Can add rocks, shells to neutralize it. You'll need to get electricity to site to hook up your pump. Look at several.you tube tutorials on how to make a backyard pond. You'll surely find one that suits you. Have fun
These will help you get started.
here are ideas/videos https://www.youtube.com/results?search_query=DIY+outdoor+ponds
You need to use a liner unless it’s very large then you would line with sand