To find a common denominator, you find the LCD (least common denominator). To start this, list multiples for both denominators (the numbers on the bottom).
2, 4, 6, 8, 10, 12, 14
5, 10, 15, 20, 25, 30
As you can see, they both have 10s. To get 10, multiply 1/2 by 5 on the top and bottom, and multiply 3/5 by 2 on the top and bottom. You get 5/10 and 6/10.
Hope this helped!