think of it as:
You have a perfectly round pizza, with some friends.
do you get more pizza if you share with 12 people or 13 people?
Oviously 12 right?
fractions are the same.
in this case: 1/12 means cutting 1 into 12 equal pieces
1/13 means cutting 1 into 13 equal pieces
Which one is bigger?
the answer: 1/12.