That depends on a lot of things. If you are in the U.S., it is probably somebody you know (at least a little bit) as this is an informal greeting.
My response is typically, "Not much, how are you"
In this context it is just a standard, informal greeting. In the U.S. the other person rarely expects you to actually tell them every minor detail of what you have done that day.
On the other hand, if you, perhaps asked somebody a question and they asked you to wait a minute. Then they come back and say "So, what's up" they are just asking you "So, what did you want?" or "So, what's going on."
These are the two primary ways that you'll see this used.
Hope that helps!