Am a 19yr old 6'8 male athlete 78kg.. wanting to gain weight and height?

No help: Unfortunately there is no way for you to increase your height. You can gain weight by consuming more calories- especially more protein and by doing strength training.

Build muscle first: Your bmi at your current weight is 18.5, at the very low end of 'normal'. Get a good physical from your pcp to be sure you are healthy and follow his advice on diet and exercise.