The tail should be left down in it's natural state, but clean like the rest of the horse, no sawdust, straw, hay, etc. should be stuck in it. Try to get the knots out too if there are any. Yes and do clean the hooves out, if you can polish them up with hoof black or clear hoof polish (depending on hoof color).
Have you watched a halter class before and know what the judge will ask you to do?