I'm gonna piggy-back this one....I second how easy it is to make your own yogurt...cuz I make my own as well!
It may seem intimidating, but once you make your first batch, you're not gonna believe it was that simple. Google is very helpful and you'll figure out what works best for you after a few batches.
Quart sized mason jars, a thermometer, a large stock pot (8+ qt), wooden spoon, tablespoon, whisk, bath towel and cooler are the supplies that get to job done for me. A gallon of whole milk and a cup of store brand plain yogurt are all the fancy foodstuffs you need. "Live and active cultures" is the more important bit.
Heat the milk to 190-195 degrees Fahrenheit. Devide among the 5 quart sized mason jars, about 24oz each. Cool to 100-110 degrees. Devide the store bought yogurt cup between the jars, about a tablespoon to each and eyeball the rest. Whisk and lid, wrap jars in towel and place in cooler. Wait 3 hours to overnight and refrigerate. Once cooled, enjoy.
The long and the short of it is you're sterilizing the milk to inoculate it with the cultures that are going to do their thing and make more yogurt.
Using Greek yogurt as a starter/culture does not yeild Greek yogure; straining of the whey with a coffee filter, mesh seive, and a bowl will get you there.
Hyzakite does have a good bit of advice, save a lil bit from the last jar...use that to make your next batch.
Feel free to reach out with questions, let us know how yours came out, too!