depending on how in depth your want to get, you need an impeller and a canister with an inlet and outlet orifice. a simple motor connected to the impeller.
For you must put minimum 2 inch gravel in pond.before putting gravel place filter board under gravel with filter board stick. stick must connected to the air pump.
Look in pond supply store and catalogs for filter making supplies. You will need a filter medium and and something to hold the medium. Typically filter use activated charcoal as the medium inside a cloth bag.
