Your sheep need two tags. Normally they come with your flock number, e.g UK 123456, on one side and an individual number on the other, starting from 0001. We put the yellow tag in the left ear and the other colour (your choice) in the right ear. Make sure you position the tag entry point into the soft tissue of the ear and not into cartilage (which you'll feel as hard lines along the ear). You'll need a set of taggers specific to your chosen tags. Some are better than others - we use Allflex. We tag all our lambs at the same time, before the first store ram lambs go to market. They need to be done by the end of the year of their birth in any case. If you have a kindly farmer neighbour maybe he'd show you how to do it the first time around. Further identification depends on which breed of sheep you keep and whether they're pedigree. The breed society should be able to give you guidance on this.