Beware not to mix up fractions (to describe a part of a whole: half, third, fourth...) and ordinal numbers (to describe an order/rank: 1st, 2nd, 3rd, 4th, ...).
For ordinal numbers, see Forming ordinal numbers (1st, 2nd, 10th...).
Look at these examples of fractions:
Note also that just like in English, the second number is plural if the first is.
Examples and resources
Q&A Forum 2 questions, 4 answers
Trois sixièmes ou un demi
I cannot think of a situation in every day life when you would use 'trois sixièmes' except in a Maths lesson...