Creating a pond

I have just started digging a boggy area of a field in order to put in a small pond. I want to create a natural bio diverse ecosystem. Looking for advice on a number of areas. 1. Should I use a lining of some sort or leave the bottom as clay soil? 2. Does it need to have a constant flow of water in and out? 3. Any suggestions on what plants I should introduce? Or should I leave nature to take its course and not introduce anything?