Sorry to hear about this social awkwardness. I can definitely relate to not knowing what to say during conversations or not knowing what to say to even start one. When it concerns not knowing how to start one, like you said, not knowing what to say after 'whats up' - Perhaps you should think of some topic that you are approaching them about, that you can say after the initial greeting. If they are in one of your classes, maybe ask them about the homework, the teacher, the subject that was talked about in class. Those are just examples (and perhaps boring ones, I admit), but I only mean that maybe going into the conversation with a starter topic would be helpful. That way you have something to say after 'whats up'. And then, perhaps the conversation would morph into something else.
Being able to hold a conversation can really seem like an "art" at times, something that we need to practice to get good at and comfortable with. I don't think that social skills are inherent in us - if we failed to develop them throughout all of our school years, then it might take some time and practice for us to get better at it. People who didn't have social awkwardness in their lives/early school years, they had more natural practice and thus, they are natural at it.
Sorry if my advice is kinda limited - like I said, I have the same type of trouble.
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