When completing Punnett Squares, you cross the letters that meet up. For example, their are two TTs and two Tts. Dominant will always show before a recessive so if it's TT or Tt it will be the dominant version. two tt's mean that it's recessive. In a Punnett Square, one box = 25%.
Part A: The genotypic ratio is 50% TT and 50% Tt. The phenotypic ratio is 100% tall.
Part B: The two Tt's could produce a short plant because there is a 25% that the parents will both give t's.