Well, first you wanna get common denominators. To do this, since 8 doesn't go into 12, multiply 3/8 by 12. The numerator will be 36, and the denominator 96. Then multiply 5/12 by 8. You get 40/96. Now add the numerators. 36+40=76. Keep the denominator. 76/96 can be reduced to 19/24.
Sorry if this was complicated. Hope it helped! ;D