There are many options on how to build your pond, and most are suitable for turtles. An easy way to get a pond is to buy one of the pre-fabricated pond shells. However, they limit the shape of your pond and might not be suitable for turtles. Plastic liner can be torn by large turtles. If you use plastic liner, make sure it is high-grade.
Concrete is suitable only if covered with dirt and rocks.
I have been told about using a combine technique of a plastic liner with a flexible mortar covering. This sounds like a good solution, if you can afford to do it. See Outdoor Hibernation for Water Turtles . Unless the pond is huge, it is recommended that you install a pond filter if there are turtles in the pond. Otherwise the water will get too dirty.