a decent sized pump is a must and id recommend a bio filter combined with a uv light to kill off any algea other than the pond liner and the gravel/sand whatever you decide to put in a substrata youl need some airiating plants and plants around the edges to supply some shade for the fish and maybe a lilly,waterfalls are optional but do tend to be a good idea as it allows the water a good airing ,depending on the type of fish your going to keep (koi tend to be distructive) you may have to put the plants into pots and weigh them down with gravel,go for as deep a pond as you can manage to prevent freezing in winter a minimum of 4ft is normaly recommended for koi ,netting for over the pond if there are trees nearby as leaves in the autumn will build up and rot giving off high levels of chemicals