For best results, secure a supply of filtered or distilled water. Store-bought bottled drinking water will work. Tap water often contains chlorine and minerals that can adversely affect the taste and appearance of coffee or tea.
Heat water to boiling in a kettle on the stove, in a self-heating electric kettle, or in a stoneware mug in a microwave oven. (When heating in a microwave oven, be sure the container doesn't contain any metal or metallic trim glazing. Remove the hot container with caution, as the water may be superheated and it could erupt violently, causing injury or blindness if it hits you in the face. This is particularly a concern when the container is very clean and smooth and the water is very pure. The hazard of superheated water can be mitigated by carefully inserting a spoon into the container at arm's length before withdrawing it from the microwave oven.)
I recommend avoiding using a glass for preparing the hot water, unless it is oven-rated borosilicate glass, as thermal stress or shock can cause the glass to shatter. Stoneware and china are safer.
Drop a tea bag into the mug of hot water and allow it to steep for about 4 minutes for maximum strenth. For weaker tea, use a shorter steeping time. Withdraw the tea bag and discard it. Instead of a tea bag, one can also use loose-leaf tea and a tea infuser.
Add milk, condensed milk, cream, or powdered non-dairy creamer and/or sugar to taste. If the mug is very full, you may need to discard some of the tea before adding milk or cream. You can leave the bag in the mug if you like, as it is already saturated with water and won't appreciably absorb milk or sugar. I remove the bag first, because I fill my mug almost to the brim and removing the tea bag lowers the liquid level enough to make space for milk.
If you choose to brew the tea in a teapot that holds more than a cup of liquid, you may need to use two or more tea bags or a large tea infuser. Some china teapots have integral strainers, allowing one to toss loose-leaf tea directly into the pot before adding boiling water.
It is also possible to brew tea with room temperature water or in a glass jar or bottle set out in the sun, but due to the lower water temperature it will take somewhat longer, upwards of half an hour to several hours. The advantage of using boiling water is that one can use water from questionable sources, as boiling kills pathogens.