Decide what type of pump you require and the correct pipe size for the amount of water you want to cascade at the mouth of the grotto. If the grotto is going to contain a room, or perhaps a hot tub it would be a good idea to provide enough waterflow to create a "water curtain" in order to have some privacy.

If you have any questions feel free to ask.
Steve

Hopefully this link may have some answers...

Here is a link that might be useful: building a grotto