Dear willdeb,
In general anything you have in your trout fly box will work for smallies particularly in larger sizes. IE if you have size 4 wooly buggers throw them instead of the size 8 or 10's. A large stonefly nymph will be eaten by a smallmouth same as it would be eaten by a trout.
If you decide you want to target smallmouth and nothing else in the stream then you do need to concentrate on somewhat larger flies. They needn't be gigantic, but size 6 and 8 flies are easily eaten by decent sized bluegills. For smallies and nothing else make size 4 your smallest fly, and don't be afraid to toss a 1/0 or 2/0.
With that in mind here are a couple "must have" smallmouth flies in size 4 to 1/0 or 2/0.
Clouser Minnow
Poppers
Sneaky Pete's or some other slider type popper
Wooly Bugger's
Zonker type flies
Crayfish, wooly buggers work as a substitute
What you want to do is cover all levels of the water column with your fly selection, topwater, mid-depth, and the bottom.
Yellow, white, black, and purple are good colors for both poppers and Clouser Minnows and Sneaky Petes.
As you get deeper in the water column you'll probably find that natural colors like tan, brown, rust, and olive work well for the crayfish and the Zonkers which can be dead drifted to represent crayfish, and eels, or leeches, or stonecats, etc.
Smallmouth can be selective on rare occasions but most of the time all that matters is putting a reasonable facsimilie of a food item in front of them at a level where they don't need to expend more energy than they will gain by eating it to prompt them to bite.
Good luck!
Tim Murphy