Do you capitalize am in a business letter?

Answer for the present tense verb:

Only if its at the start of a sentence. Example: Am I assigned to bring something to the picnic?

Answer for AM, abbreviation for the time between midnight and noon:

The abbreviation is expressed as a.m. or AM. Example: The train leaves at six AM. The train leaves at six a.m.

For all acronyms or initials, it is capitalized: AM or A.M.