The letter A is signed by holding up your dominant hand in a fist, facing outward, with the thumb sticking up to the side of the fist. This looks like the small letter 'a', but upside down, where the thumb sticking up reminds you of the serif sticking out the side of the small letter 'a'.
Teach your toddler the letter A sign when it is necessary to finger-spell some simple words - or the initials of objects, places, animals, or people's names. That way, your toddler slowly begins to associate the sign with the sound of the letter in a word or name.
This is a useful flash card taught best in conjunction with the other letters of the alphabet.