A "starter" contains the live bacterial cultures that help transform milk into yogurt. You can start a batch of homemade yogurt two ways: from a few tablespoons of store-bought (or previously homemade) plain yogurt, or with a yogurt starter powder. If using store-bought yogurt, pick a plain yogurt (regular or Greek should work fine) that tastes good to you and check the label to verify that it has live, active cultures (this part is very important). Also check to make sure the yogurt doesn't contain flavors or added sweeteners (like sugar, Splenda, or Aspartame). It's also best to avoid a yogurt that uses additives or thickeners (like pectin, gelatin, or dry milk powder).
One can choose any type from skim to whole milk. Whole milk makes thicker, milder yogurt as compared to skimmed milk.