If you brew it hot using the method on the box, you'll have to refrigerate it afterward. Otherwise, the ice in your glass will melt quickly and your tea will be relatively dilute. I have never tried brewing it cold. I can't imagine it would work very well, unless you use the tea bags which are specifically made for cold brewing, but those are made with really low-grade tea (not that regular tea bags aren't made with relatively low-grade tea, but cold-brew bags are, especially). I brew my tea double strength, then dilute it with an equal amount of ice/water.
For example, if I need 64 oz. of tea, I brew 32 oz, but with the same number of tea bags as would normally be used if brewing 64 oz. Then, I pour it in the pitcher and put ice in it a little bit at a time, while stirring, until it reaches the 64 oz. mark (actually, not quite to the 64 oz. mark, since I like mine a bit strong). This method produces tea that is cold immediately and is of the proper strength. One downside is that the tea does not look particularly clear when poured, but that has no effect on the taste. Another is that the ice, which may have been sitting in the freezer for a while, may have absorbed some of the odor from the air in the freezer, but that's usually not a problem if you have baking soda in your freezer. Not to mention, you're going to put ice in your cup, anyway.
I usually drink mine unsweetened. You'll have to experiment with different amounts of sugar to find your own sweet spot (pun definitely intended). Be sure to add the sugar before you add the ice. Getting sugar to dissolve in cold tea is a real pain. One cup of sugar per 64 oz. will yield very sweet tea.
If you're using tea bags, Luzianne is pretty good. It's less than twenty cents more expensive than the Walmart brand for a 24-pack of the "family size" bags, and tastes much better.