Technique and strength
The notes above are right that technique is a big part; however, simple strength always helps. I recommend doing some long toss with a baseball or softball. Get a friend, sibling, or parent and a couple times per week work on throwing from farther and farther distances. Increase gradually because throwing a lot can quickly lead to an injury. However, after 6-8 weeks of diligent throwing, you'll build up arm and shoulder strength. This is a technique used by many baseball pitchers to build up their arms.