The common factors for the beginning-intermediate fly fisher in fly selection are commonly stated as size, color and shape for trout.
For the beginning fly fisher, the best advice I can offer is identify size as best as you can (usually smaller than you would think, save those Drakes and Hex hatches). Then color and then shape or stage. This is important whether fishing a nymph, larva, pupa or dun/spinner.
Study up on the hatches for the bodies of water you intend to fish and then study those hatches for size, variances of color and overall shape.
Also, not to be forgotten is action/presentation. This can be movement to suggest life or lack of movement, but presenting the fly, untroubled, into the zone. Fly above: I tied a smaller version of the Zug Bug.
