Simply speaking, disgust is the response we have to things we find repulsive. Some of the things that trigger disgust are innate, like the smell of sewage on a hot summer day. No one has to teach you to feel disgusted by garbage, you just are. Other things that are automatically disgusting are rotting food and visible cues of infection or illness. We have this base layer of core disgusting things, and a lot of them don’t seem like they’re learned.
But there’s also a whole set of things that have a lot of cultural and individual variation about whether it’s considered disgusting. For example, I like bloody steaks and my girlfriend, who is a vegetarian, finds them repulsive. The core base of what causes disgust has expanded to the point where certain kinds of moral violations, social transgressions, and even value systems of groups one is not a member of can come to be disgusting as well.
There’s a good case to be made that the culture we grow up in can fine-tune our disgust response or calibrate what we come to be disgusted by, but people don’t really need to learn how to be disgusted. The reaction is specified by nature, although it doesn’t start until we are around 3 or 4 years old.