show'st = showest = to show
EX:
Please show me the way to the post office.
grow'st = growest = to grow
EX:
The kitten grows bigger everyday.
I'm not recalling which sonnet "th'account" appears, but, knowing Shakespeare, it would mean "the account". This would be because of "the" and "account" both starting with the "uh" sound. Instead of speaking a double-uh sound, it would be blended into one: th'account.
Hope this helps because I love Shakespeare's work! Good luck. :)