You can use nonfat greek yogurt, or plain non-greek yogurt, just note the mouthfeel and texture of the yogurt ranch will be slightly more sour and tart. I recommend adding an additional 2 tablespoons of mayo if you’re not using a full fat yogurt.
If you’re looking for a more traditional ranch flavor and texture, I recommend making my classic homemade ranch dressing.
Add the milk if you prefer a thinner consistency. For dips, I find you don’t need to add the milk.