Many cocktails need to be sweetened, simple syrup is the best way to do so, and it's also the cheapest way, because all you need is sugar and water.
This simple sugar syrup is also a must for making sweet and sour mix, this mix is essential for many of the most amazing cocktails in the world.
In a sauce pan, put 4 cups of sugar and 2 cups of hot water (or any 2 to 1 relation according to the amount you wish to make), stir and allow the sugar to dissolve, wait until you get a don't let sugar crystallize!
You can also use:
Powdered sugar can be added to any cocktail or sour mix instead of simple sugar syrup, although I always prefer liquid over powder
Most liqueur stores offer pre-made simple syrup, but there is nothing like making your very own