Reading and interpreting Serial information
Two things immediately jump out at me:

char FirstLetter = input.charAt(1);


charAt(1) is the second letter, not the first - the first is charAt(0).

if (FirstLetter = 'U')


That is saying "If I have been successful in assigning the value 'U' to FirstLetter then..." - "=" is assignment, you need "==" for comparison.

Also your readSerial() function is complete junk.  It's not doing anything at all like you think it is.  You should read this to better understand Arduino serial reading: http://hacking.majenko.co.uk/reading-serial-on-the-arduino