Find the greatest common factor of the denominators, so in this case, it would be 12. To get 4 to 12, multiply it by 3. Because you multiplied the bottom by 3, do so to the top number. Now you have 9/12.
To get 3 to 12, multiply it by 4 and multiply the top number by 4 as well. Now you have 4/12.
Add 9/12 and 4/12 to get 13/12. 13/12 is already in simplest form, but can be made a mixed number. This would be 1 1/12.