It would be great if you added some more details, but for common denominators you try to find the least common number that the 2 or more of the numbers you got have!
For example:
1/2 and 2/3 if you multiply 2 by 3 and 3 by 2, they both are 6, so you would multiply 1/2 both by 3 to make it into 3/6 and 2/3 by 2 to make it into 4/6!