I recommend tying three dry flies in the largest size (10 or 12), same pattern but you can vary colors, then three in each smaller size you are planning on tying down to the smallest you think you will need. You will probably think an 18 is as small as you want to go for now but someday you will want to tie even smaller sizes. Repetition is the key to getting better and once you've tied the biggest fly it is easier to tie a smaller size. I'd include a parachute pattern to those previously recommended, my favorite for Brook Trout in the Shenandoah Mountains in Virginia is the Mr. Rapidan Parachute; it has a yellow calf tail hair post that makes it easy to see on the water.