I use organic half and half or cream, or cream line milk, or whole milk. Anything less than whole milk makes a really runny yogurt. I prefer milk from a farm, with as little processing as possible. I have discovered the hard way and then had it confirmed by research, that ultra pasturized anything will be hard to turn into yogurt. So if you do get store bought milk, try to find one that does not say "ultra" anything.
I take about a quart of milk (or cream) and heat it on the stove (stirring occasionally) to about 180 degrees or until it starts to climb the sides of the pot. It should go without saying, but all of your cooking utensils should be really clean before you start. It would suck to add some random crummy bacteria into your yogurt.
While that is heating up, put two tablespoons or so of starter yogurt in a glass measuring cup.
This is to bring the yogurt to room temperature. You can use any yogurt, as long as it is not flavored. I prefer to use either a little of the yogurt from the last batch (which I never remember to save) or plain thick yogurt from the farmers market. I'll use Trader Joe's whole milk plain yogurt as well. I always use whole milk yogurt.
Once your milk reaches 180, pull it off the heat and let it sit and cool. I let it go to about 95-100 degrees.
Pour the yogurt/milk mixture into your yogurt cups - my yogurt maker came with glass cups which again makes it easy. Load the cups into the yogurt maker and turn it on.
I leave mine for about 10 hours. Any less and it is too runny for me. I usually start yogurt at night after the kids are asleep. Then I get it into the yogurt maker before I go to bed and it is done when I get up. Once it is done, put it in the fridge with lids on the cups for a few hours - 3 or 4 at minimum.
I usually add fruit or nuts (if I add anything) when I serve it rather than try to flavor it before it gels. I would like to try adding some vanilla bean as it cooks, but I just haven't gotten to it yet. My kids eat this like crazy and it is not a sweet yogurt - it actually has quite a tang to it.