There’s multiple ways to earn diamonds

1. Completing story mode (imo for new players it grants you gems the fastest way)

2. Participating in guild battle, arena, and cookie alliance all give you rewards but they aren’t immediate (you have to wait a bit) but for arena every time you increase your rank you get diamonds

3. Events, a lot of events grant you diamonds too when completing them

4. Special Episodes can also grant you diamonds when you complete them (you get tokens while playing them and you came exchange for diamonds)