Are you a member of the Shetland Sheep Society? There's endless info about Ram selection there.
Basically you can't decide if a lamb will turn out to be a good breeding flock sire when he's only a few days old, or even a few months old. You can see that he has promise, but that's not quite the same thing.
When we kept Shetlands, we just castrated all our male lambs and bought in tups from well known flocks. There are all sorts of criteria to look at when choosing a tup such as conformation, lineage, mouth, amazing fleece, testicles, strong back, good feet, majestic stance etc etc. As beginners you have no hope of deciding if your own lambs are suitable. Add in that any one you choose will be related at least to his mother but also to all the other lambs if you used one tup, then you are far better to buy in a tup for next breeding season. In the meantime, go to every show with Shetlands there that you can, watch which do well and try to work out why, for yourself, then start speaking to those showing. Shetland sheep owners are always desperate to point out their own animal's good points so you will learn plenty. Expect it to take a few years before you can pick out the Champion before the judging