In general, you'll want to use about 2/3 the amount of coffee you would use for a hot cup of coffee. So, if you normally use 2 scoops (tablespoons) of coffee for a 12 ounce cup of hot coffee, you would use 1 and 1/3 scoops for an iced coffee.
Of course, this is just a starting point. You may want to experiment a bit to find the perfect ratio for your taste. And keep in mind that the amount of coffee you'll need will also vary depending on how much ice you use.