First you find a common denominator, then you convert both fractions to the common denominator.
For the common denominator, find a number that is divisible by both denominators. Just multiplying both will work, but this is not always the most efficient method, since this may cause you more work later, to simplify the result. Once you decide on a common denominator, in both fractions multiply the numerator and the denominator by the same number.
Example: 1/3 + 1/4
Common denominator is 12
Multiply the first fraction by 4/4, and the second by 3/3, to obtain:
4/12 + 3/12