Here is a good link to some info on getting started. A Guide to Garden Ponds. Resources, Tips, Maintenance, Construction Guide, Pond Installation, Resources

After landscaping in many ponds and waterfalls over the year, I have found that more pump than needed is always the way to go. Also depending on the size of the pond itself, you may want to consider having a pump for filtration and a separate pump to supply the waterfall.

Pump size will depend on both the height the water needs to be moved up to, and the width of the waterway the water will flow down.