French possessives agree with the thing owned, not with the owner: mon before a masculine noun, ma before a feminine one, mes before a plural. Your works the same way (ton, ta, tes; formal votre, vos), and so do his and her: son, sa, ses - so sa mère can mean his mother or her mother.
Examples
- Mon père s'appelle Marc.
- My father is called Marc.
- Ma sœur habite à Lyon.
- My sister lives in Lyon.
- Mes parents sont à la maison.
- My parents are at home.
- Comment s'appelle votre fils ?
- What is your son called?
The mistake to avoid
Before a feminine word starting with a vowel, ma becomes mon: mon amie, never "ma amie".
Where this fits in
Family & people, French A1: Talk about your family, say what people are like and how old they are, and introduce your friends.