Look at what common number the denominators both divide into. For example if your denominators were 2 and 3. Your common denominator would be 6. Because 6 is the smallest number that BOTH 2 and 3 divide into. 12 would also be a common denominator but you alsways want to use the smallest common denominator. Here are a couple of other examples
1.) 4,5 LCM = 20
2.) 6,12 LCM = 12
3) 5x^2, 8x LCM = 8x