Making an herbal brew by steeping herbs in hot water is known as an infusion. Its basically what you are doing when you are sticking a bag of lipton tea in a cup. Except herbalists generally use more herbs than you usually find in a tea bag.
So how much should you use and how long should you infuse the herb for? This really depends on the herb and your needs (how strong or weak.) Generally flowers require only a short amount of time to steep and some really vitamin and mineral rich herbs like nettles and oat straw can be infused for quite a long period of time. Take a look at each herb separately and read the dosage instructions for each herb.
Some general ideas:
Flower teas: (Examples are chrysanthemum, chamomile and elder flower). Flowers are pretty delicate and release their constituents pretty easily. Steep 1-2 teaspoons per cup of hot water for 3-5 minutes while covering the cup to prevent precious volatile oils from escaping. Then strain and drink. If the flowers are whole, you might want to add about a tablespoon.
Leaf teas: (Examples: spearmint, holy basil, gingko). Leaves can be steeped a little longer but the longer you steep them the more the bitter principles will come out. Generally I advise steeping 1-2 teaspoons of dried leaf per cup of hot water for about 10 minutes while covering the cup. Then strain and drink.
Long Infusions: These are herbs that can handle really long infusion times (think 4-12 hours) and not become bitter and unpalatable. This is generally done when you want to work with herbs to get a lot of nutritional value in the form of vitamins and minerals. Examples include oat straw, nettles and red clover. See Nutritional Tonic Recipes for examples.
Cold Infusions: This is generally done with herbs that are slimy (think marshmallow, slippery elm) or those that have delicate essential oils (think lemon balm). Often cold infusions are done over a long period of time, from 4 hours to overnight. A classic marshmallow preparation would be to add marshmallow root to 1/4 of a jar and then fill the rest with cold water and steep overnight. IN the morning, strain and drink.