Simple. if the fractions are similiar fractions (meaning to say, they both have the same denominator) no need to find  the LCD . just simply add their numerators then copy the common denominator.

but if the fractions are disimmilar fractions (different denominators)
find its LCD.

How to find the LCD?
think of a number that u think is both factors of the denominators of the given fractions and then divide it first to the first fraction then multiply the qoutient to the numerator and continue the steps in the next fraction. perform your answer through the indicate operation.