If you've ever made any sorta of beverage - a cocktail, iced tea or coffee, lemonade, etc - you've learned some basic science - sugar does NOT dissolve in cold liquid with a simple stir. It takes either 1) LOTS of agitation or 2) a warmer liquid. Sometimes, this is a good thing, as granular sugar is often a key ingredient to a drink that involves crushing or muddling, like a mint julep or a mojito. And sometimes, it's really annoying and inconvenient.
So, the beverage-world has long used a liquid sweetener known as "simple syrup" to incorporate sugar into drinks. It uses a basic "simple" ratio of 1 part sugar to 1 part water. Some approaches call for as much as 2:1 sugar to water, which increases efficiency, but I'd keep it 1:1, or at most 1.5:1. See, simple syrup provides not only sweetness to a drink, but also volume. AND, most cocktail recipes will presume a sugar level of 1:1, so it's best to stick with what the pros use.