how to make yogurt unsweetened

As mentioned, your yogurt might contain an additive that's making it sweet. Even if not, some yogurts are tangier than others. I'd say to try several different brands of plain yogurt to find which has the tang you like. (Also, taste is very subjective - perception of sweetness, spiciness, saltiness and so on can vary widely.) I generally find sour cream to be milder, not tangier, than yogurt.

I don't understand this either. Plain unflavored yogurt is naturally tangy and I use it in place of sour cream in baking and dips all the time. Check the ingredients--there should be only milk and maybe pectin. If not, and there is sugar or something, you have flavored yogurt and you can't use it for dressing. Otherwise, if it is not tangy enough, as someone said you can add a splash of lemon juice and vinegar would be fine for a dressing, white wine vinegar would be good or any that you have should be fine for dressing.

The lactose in milk is a naturally occuring sugar. sour cream contains lactose as well, so it also has an element of sweetness. Unless you have vanilla or some other flavored yogurt, you should be able to substitute the yogurt with no problem.

I've never thought of plain yogurt as sweet. Are you sure it isn't vanilla yogurt? You could add lemon juice or buttermilk to increase the tang.

Depends on the recipe. What are you making?