I'm from the South of the United States, so I know from iced tea. I recommend Red Rose brand tea, which is available in the U.S. and Canada. In England, I recommend PG Tips or Ty-Phoo, because there is no reason to spend a lot of money for iced tea. If you can't find these brands, I suggest you use a mid-priced tea, preferably a blend of black teas, such as English Breakfast blends. (Warning: in the States, 'English Breakfast' tea is usually over-priced! Just use Red Rose or a generic supermarket brand like "Crown Colony.") However, I would recommend that you not use Lipton; any kind of instant tea; or Celestial Seasonings, some flavors of which taste like Endust when chilled.
You will need a teacup, tea in a bag/infuser/ball, boiling water, and a strong glass, or large plastic cup, full of ice.
Brew your tea in the teacup so that it is slightly stronger than what you usually drink. Add sugar or honey if you prefer a sweet tea (read this first). Pour the tea into the glass of ice and stir it with a spoon until it is chilled. Add more ice if the cubes melt completely.
Add a slice of lemon or sprig of mint to the glass.
You will need four to six portions of tea in bags/infusers/balls, a saucepan or kettle of boiling water, a plastic pitcher, a knife, and a refrigerator.
Brew up your tea very strong in the teapot or in a saucepan. Use at least one more teabag or teaspoon of loose tea than you usually use. Brew for 20 minutes or so. Do not let it sit longer than an hour. Put a lot of ice cubes into a pitcher or beverage container (say, two or three trays' worth) and pour the tea over them. Serve the tea from the pitcher or store in the fridge.
The resulting tea may be too strong for some tastes. Serving it over more ice will help dilute it; if that's not enough, simply add water when serving. Add a slice of lemon or sprig of mint to the glass.