Tomi, as long as the Kombucha you are buying is NOT pasteurized and has the live Kombucha cultures, you can use it as a starter, and it will grow a "mushroom" (symbiotic colony of bacteria and yeast) disc on top of it as it brews.
To make one gallon of your own Kombucha drink, you will need a wide mouth glass jug (please do not use plastic or ceramic, it can leach harmful stuff into the kombucha!) You can often buy suitable ones at Wal-Mart with the food storage supplies (I know, I hate shopping there... but there's nowhere else in my town that has nice kombucha-size glass jars!) Some people use a gallon pickle jar. Paper towel or very clean cotton cloth to cover the mouth, rubber band or elastic to hold the cloth cover on the jar. Black and green tea bags, sugar, and 1-2 cups starter. (Don't use honey to make kombucha. White sugar actually works very well, believe it or not, while brown sugar can give the brew a funky flavor that not everyone cares for. The sugar is there to feed the bacteria and yeast, and they convert it into healthy stuff.)
Boil one quart water. Put in 4-5 bags black tea. Let sit for one minute, than add 4-5 bags green tea. Let the tea steep for 5 minutes, then take out the green tea bags. Take the black tea bags out after 10 minutes total steeping time. Add 1 cup sugar, stir until dissolved. Pour the tea concentrate into your brewing jar, add cool filtered water until the jar is almost full, but LEAVE ROOM for your starter (and in future batches, you will also need room for your "mushroom" thingy). Check to make sure the tea is no warmer than room temperature. Too hot will KILL your starter. If it's too warm, cover it and let it stand until a suitable temp has been reached. Add starter, cover with a cloth and DO NOT DISTURB for at least 5-7 days. At the one week point, take a taste. It should taste tangy, slightly sour, and have a flavor that is similar to apple cider, but different. It will probably still be pretty sweet at this point, and it should have at least a thin film of stuff floating. This will become your "mushroom" for helping start the next batch. The longer you let it set, the more tart it will be, but the more active cultures it will have. The key is to find the balance of benefits and drinkability. It may take a few batches until you find the balance that works right for you. Once your first batch is ready, pour it out into glass jars for storage, reserving some liquid for starter to be added along with the "mushroom" pancake looking thingy to the next batch.
In my kitchen, Kombucha is ready to drink after about 10-12 days depending on how hot it is. It brews faster in summer because of the warm weather.
momma2annabelle, I think it's fine to drink kombucha whenever! Just use common sense, good judgement, and listen to your body. Start off slow, with a few ounces and see how it makes you feel. Make sure you are drinking plenty of water of course. If a few ounces sits well, and you want to drink some more, go for it. If it doesn't seem to do well for you, then listen to your body and wait. My kids actually really like it. I wouldn't recommend drinking a whole quart of it everyday, the most I ever drink is about 12 oz. total spread out through the day.
Kathryn