To make yogurt, you will need a starter culture, milk and a way to incubate it at about 100 degrees F, for 12- 24 hours.

Starter culture can either be bought in dry form in small packets or started from an existing live yogurt culture.

Mix the packet of culture into warm milk (about 80 deg. F.)  Set in a warm place (100 deg. F) for 12-24 hours.
Buy your favorite brand of yogurt (plain flavor) and mix 1/4 amount of the culture (yogurt) to milk.  Example, 1/4 cup of yogurt to 3/4 milk = to make a cup of yogurt.  Set in a warm place for 12-24 hours.

To make a cheap, easy to use incubator, use a themal cooler (like you take camping or carry a lunch in). Put in a heating pad set on low, or fill containers with hot tap water to set next to your cultured yogurt container in the cooler.  If you use jars of hot water, you will need to refill them every few hours to maintain your 100 deg. F temperature.

Some people incubate thier yogurt on the top of their fridge or on their hot water heater.

Homemade yogurt will not be as thick as store bought yogurt as they add thickeners.  To have yours thicker, you can pour off some of the whey that rises to the top (a clear, greenish, liquid).  The left over curd will be a little more solid and thick.

After you get a good yogurt starter going, always remember to keep some back to make your next batch!