Bonjour !
The cedilla is used to change the "K" sound into an "S" sound.
It is necessary when you need to keep an S sound in front of a hard vowel: a, o, u.
Exemple :
- ça
- garçon
- gerçure
- glaçon
The consonant C naturally sounds S in front of a soft vowel: e, i, y
Exemple :
- merci
- ce
- celui
- cependant
- commerce
- citoyen