Warm the starter. The starter is the bacteria that you add to the milk, which in turn cultivates more bacteria--bacteria that is necessary for the creation of yogurt. Let the starter yogurt sit at room temperature while you're waiting for the milk to cool. This will prevent it from being too cold when you add it in.[2]
All yogurt needs "good" bacteria. The easiest way to add this is to use existing yogurt. The first time you make your own yogurt, use store-bought plain (unflavored) yogurt. Be certain it has "active cultures" on the label. Taste various plain yogurts before you get started. You will find that different kinds taste a bit different. Use one you like for your own starter.
Alternatively, instead of using existing yogurt, use freeze-dried bacteria cultures (available in specialty stores or online), which are more reliable as a starter.
In a pinch, you can use a flavored yogurt, but the taste of the resulting cultured yogurt will not be exactly the same as using plain yogurt.
Any good tasting sour cream can be used, particularly if you do not wish to have ropey or stringy strands of bifidus (usually found in commercially made and thickened yogurt for its ability to stand up to the processes and still be an aid to your digestion). If using a bifidus culture, mix in with a sterile blender to obtain the proper distribution with the proteins in the milk. If you still have stringy ropes you may be heating your scald too fast or too long, use a double boiler in that case. At high altitudes this can be more of a problem.