A momoir is a story about you and your mom.
It can be an episode in your life you remember vividly -- something that was fun, sad, poignant --- or an everyday occurrence, one that in the reliving informs you about how your mom helped instill values in you, or had fun doing something simple like playing a game with youf. Or reading to you, and how perhaps that helped set you on a course of enjoying reading for the rest of your life.
It can also be a piece about your mom's story ... where she was born, what she did in life, her own joys and accomplishments, obstacles and how she overcame them.
Remember that photos will enrich your story immensely.