Nouns ending in -er are usually masculine, but la cuiller is feminine.
Cuiller is one of the rare exceptions: learn it as it is.
Is cuiller masculine or feminine?
lacuillerexception
The rule behind it
94.6% of -er nouns are masculine
In context
Comment est-ce que je mange, sans cuiller ?
How do I eat without a spoon?
Il y a une cuiller.
There is a spoon.
J'aime les cuillers.
I like spoons.