Brewing tea is more of art than science. There are many variables that will affect your final cup: the tea, steeping method, water quality, water temperature and time.
Always start with fresh cold water. Heat the water based on the type of tea you are drinking. Generally, water for black and herbal teas should be boiling (212F) and water for green teas should be considerabley cooler (170-180F). Steeping time varies based on the tea and your taste, but 2 to 5 minutes should be the range for green and black tea bagged teas. Typically, herbal teas can be steeped for at least 5 minutes.