The simple and literal answer is that two things happened after Jesus called Levi to join him.  First, Levi got up, left everything, and went with Jesus.  Second, Levi threw a big party for Jesus at his house and invited all of his friends to come and meet Jesus.

I assume, though, that you are expected to think about the significance of what happened, not just the simple facts.  Both of the things that happen when Jesus calls Levi are significant.  First, Levi leaves everything and joins Jesus.  This symbolizes how people are supposed to follow Jesus without reservation.  They are not supposed to worry about how the world will perceive them or about any other worldly issue.  They are simply supposed to follow him.  Second, Jesus is willing to eat with Levi’s friends.  This is important, particularly in the context of this chapter where Jesus has just touched a man who had leprosy.  The point is that Jesus is willing to mingle with the people who are considered unclean both in a physical sense (the leper) and in a social/moral sense (Levi’s friends the tax collectors).  This shows that Jesus has come to minister to people who are rejected by traditional society.  He has come to attend to the “sick” not just to the “righteous.”

Thus, what happens after Jesus calls Levi is important for what it says about who Jesus is targeting in his mission and for what it says about how we should accept Jesus fully and completely.