First, find the common denominator.
The common denominator of 12 and 9 is 36.
Therefore, you multiply 12 by 3 to get to 36, and you always do the same with the numerator. Meaning, you multiply 1 by 3 as well.
This makes 1/12 into 3/36.
To get 36 from 9, you multiply 9 by 4. You do the same with the numerator, which is 5.
This makes 5/9 into 20/36
Now that you have the common denominator, all you have to do is add the numerators.
3/36+20/36= 23/36
Your final answer would be 23/36
Hope this helped!