How do you get rice balls to stick together?
Also add a good bit of salt to the water. Then, keep a bowl of cold water with salt and coat your hands between forming. This will stop the sticking. Next, the filling can be something dry, like skipjack tuna w a dab of soy sauce or furikake (rice seasoning).
Why did my rice ball fall apart?
If you are using long grain rice (such as jasmine rice), the onigiri will simply fall apart because they are not sticky enough. You can overcook the long grain rice so it will be soft and mushy to form a shape, but it certainly will not taste very good.