My future ambitions is to attend Charleston Southern University and major in Psychology.
My ambition is to attend Charleston Southern University and major in psychology.
 Delete the word future. It is not a future ambition, it is an ambition you have right now.  Remove the s at the end of ambitions, you only have one ambition in this letter. I would say goal instead of ambition, but that's just my preference. Sometimes, to some people, ambition has a negative connotation.  The names of school subjects are common nouns and should not be capitalized. The exceptions are when proper nouns are part of the subject's name, such as English composition, or Russian monarchs; or when you are referring to the exact title of a course in which case you use the standard rule for capitalizing titles, The Psychology of Human Beings in High-Stress Situations.