You will want to do this outside, or in a place where you can clean up easily -- scales will fly everywhere. I learned this the hard way: I scaled many fish in an old apartment of mine, only to find scales attached to the ceiling when I moved out, years later.
The easiest tool to scale a fish is a butter knife. They make special scalers, but you don't need one.
Start scaling your fish by holding it firmly with one hand and scraping from tail to head with the knife.