The fractions you start with have to be re-written with the same (common) denominator.
For example, if you were asked to add 3/5 and 6/7, you would:
1) Identify the Lowest Common Denominator (LCD): Here it is 5*7 = 35.
2) Multiply both numerator and denom. of 3/5 by 7: 21/35
3) Mult. both num. and denom. of 6/7 by 5: 30/35
4) Combine these two equivalent fractions: 21/35 + 30/35 = 51/35 (answer)
Try a few such problems yourself and write back if you need further help.