You need to get the denominators (the numbers on the bottom) the same. Eg.
2/3 + 1/5
I'd I can convert both to be the same type of fractions the sum is easy. By making them 15ths (by multiplying the denominators) this will work.
I need to convert 2/3 into 15ths - simply multiply top and bottom by 5
2/3 = 10/15
I now need to convert 1/5 into 15ths by multiplying top and bottom by 3
1/5 = 3/15
my sum is now 10/15 + 3/15 = 13/15