What will happen to me if I Fight?

Ok so I just moved out of state, and as we all know people change. Well this guy who used to be all quiet and shy thinks hes a bad ***. he decided to show off and call my sister a slut on facebook. I wrote on his wall and told him to watch what he says to my family. My friend told me that when i come in town to visit my dad on friday that him and a friend planned on jumping me. im a lot bigger than both of these kids and im not worried, but one of my friends wants to go with me to keep it fair. What will happen if i fight him and hurt him. I wont hit him first.