What do you say to make a horse go left?

I don't quite understand your Wording...

But, guessing that we are talking about turning horses, to turn a horse you can use your reins and legs. With your reins: if you want to turn left, you take the left rein and pull it left. You can reinforce it with leg aids, by squeezing the opposite leg. For example, if you want to go left, squeeze with your right leg. If you want to go right, squeeze with your left leg.

All horses are trained differently, though, so they might have different cues (for example, some respond only to voice). However, in general, this is what you do! :)