Please credit Sketchy77 from TrueAchievements.com for the following solution:
This can easily be completed on the tutorial map.
After completing/skipping the tutorial, head up the cobblestone stairs and into the second room to your right with bookcases and an enchanting table in it. Access the chest on the right and take out an iron pickaxe.
Now leave the building and turn right and walk towards the large building where you will see some with the pumpkins and 4 iron blocks. Mine the iron blocks with your pickaxe.
Take these blocks to a crafting table, and break them down into ingots. Now create some shears.
From there, simple head down the stairs opposite the enchanting table room and you'll see a bunch of pens with animals in them, find the sheep pen and shear one.