Before choosing an adjective possessive in French, you must first ask the question, who the object belongs to. 

Example : If the item belongs to 'je' , then we use the following possessifs adjectifs : mon / ma / mes

Then, you need to determine if the item is masculin, feminin or plural.

Mon : to complement the item which is masculin and singular 

Example  : mon stylo, mon crayon

Mon : to complement the item  which is feminin and singular that starts with a vowel or silent 'h'

Example : mon amie , mon étudiante

Ma : to complement the item which is feminine and singular

Example  : ma robe , ma voiture, ma gomme

Mes : to complement the item  in plural either masculine or feminine 

Example  : mes chats, mes étudiantes, mes gommes