A letter of the alphabet in a dream often represents communication.
Since the letter A is at the beginning of the alphabet, a dream in which the letter A appears can signify a need to initiate communication.
In general, the letter A can represent the beginning of something.
The shape of the letter can also be important. The two legs at the base of the A can symbolize support. The point at the top of the A can be a pointer or arrow that is leading you in the direction of something.
Other letters in dreams have different meanings. An O for example is a circle, signifying completeness.