Nouns ending in -in are usually masculine, but la fin is feminine.
Fin is one of the rare exceptions: learn it as it is.
Is fin masculine or feminine?
lafinexception
The rule behind it
97.4% of -in nouns are masculine
In context
Je l'ai entendue jusqu'à la fin.
I heard her to the end.
Il essaya de mettre fin à leur dispute.
He tried to put an end to their quarrel.
Oh, je n'ai pas aimé la fin.
Oh, I didn't like the ending.