They have the same denominator in this case so simply add the numerators:
10+8=18, then divide by the denominator: 18/12, then reduce the fraction: 3/2 because 6 is a common factor.
In cases where the denominators are different you need to find the lowest common denominator (LCD, a number which all the denominators divide into exactly). Sometimes all that's needed is to multiply the denominators together and use this product as the common denominator. The LCD is the product divided by the highest common factor (HCF) of all the denominators. The fractions can then be represented as multiples of the fraction 1/LCD. Then it's just a matter of applying plus or minus, as required, to the numerators. This sum divided by the LCD is the result of adding/subtracting the original fractions.