First you have to convert the denominators to the Lowest Common Multiple (unless the denominators are equal, of course).
Example 1/4 + 1/2
The LCM of 2 and 4 = 4 so we can write 1/2 as 1*2/2*2 = 2/4.
Now we can add:-
1/4 + 2/4 = 3/4.
Same thing goes for subtraction.
Example 2:-
5/6 - 1/7 :-
LCM of 6 and 7 = 6*7 = 42 so we have:-
5*7 / 6*7 - 1*6 / 7*6
= 35/42 - 6/42 = 29/42