Re: Bald spot on back of the head...
Have you ever seen a man with a "comb over:, "caesar", or other attempt at disguise? It always looks worse than a bald spot. I can only imagine how rough it is to be 18 and balding but with nicely combed, clean hair and being shaved and smartly dressed you will look much better than some attempt to cover up the spot. If you try to paint it with makeup and someone comes up behind you (unless you are really tall) it will be obvious. Maybe the best thing to do would be a self-tanning cream. It will darken your scalp somewhat making the bald spot less noticible and shiney. Up close will look like the sun did it.
Have you considered Rogain? I started that in my late 30s and it stopped my hair loss. I wasn't bald but was thinning and it stopped the thinning. At 18 it maight actually grow your hair back.