When using raw milk to make yogurt, there are several factors to consider. Once you've decided to use raw milk, however, the easiest culture to use with raw milk is a single-use, or direct-set, starter culture.
Direct-set yogurt cultures are one-time-use cultures. Either one of the following can be used as the starter culture:
A packet of freeze-dried powder that is stored in the freezer and used to inoculate each batch OR
A small amount of yogurt from the store. If using store-bought yogurt as starter, be sure to use an unflavored variety that is labeled "contains live active cultures."