Force = mass x acceleration. In this case force ( how hard you hit) will be improved by either increasing your mass I.e gaining weight, provided that your strength to weight ratio remains the same . Or by improving your acceleration, in this case your explosive power, done by improving your leg strength. Which you can accomplish through increasing your deadlift and squat. Some people will tell you plyometric exercise, but until you have a double bodyweight squat, the risk reward of plyometric stuff makes it a poor choice for improving.
Make sure you wrap your arms round their legs firmly (ring of steel) and when your tackling them make sure you drive off your legs harder, also smack them really hard with your shoulder. It feels so good doing a hard hit. The key thing in a hard tackle is staying down low and carrying on driving because thats where all the power comes from.
