Clean a fish

First, wash the fish using fresh tap water.
Step 2 Remove scales by scraping the fish with the dull edge of a knife. Hold fish tail firmly with one hand with other hand hold knife. Use short strokes from tail to head, scaping knife along fish to get all the scales to come off , turn fish repeat on other side. The sides of the fish up to the gill area are fairly easy to scale but care should be taken to remove all the small fish scales from the underbelly, head area and close to the fins and gills. If you plan to skin the fish before cooking, this step is not necessary.
Step 3 Make a cut from the gills to the vent.
Step 4 Remove the entrails with your finger. Pay particular attention to the area near the backbone and remove any blood with a small spoon if necessary. Check the cavity carefully to make sure everything's been removed. Rinse the cavity well.
Step 5 If cooking the fish whole, remove the dorsal fin by cutting along each side of it and carefully pulling the fin out.

http://capmel.com/how_to-fillet_fish.htm