Here are some websites with good ideas for in-ground ponds and above ground ponds. Remember mature female aquatic turtles need access to land to lay eggs in even if they have never been around a male. Almost all aquatic turtles will also need a basking area where they can get completely dry.
Before adding plants to your pond, be sure they are not toxic to turtles. A link is included below that will take you to toxic plant lists.