HELP ME URGENT !!!!! IN TrOUBLE !!!!!!!!?

my mom bf had his son come over and stay for a few months now i know i was wrong but my mom and her bf went to dinner so they left me and his son (cj) at the house so anyways he was useing my mom's cell and my grandma called on the phone so i heard the phone ring and i saw cj he was talking to her and i didn't know it was her then he said it was my grandma so anyways i got the phone and reached over and hit him on the head not like it was a hard hit but like if u had a mitt and a small hit on the head but he had his cap on so he didn't feel it to make a lon story short he comes and runs over to me and hits me in the face i know i was wrong but i was teaseing not like i was trying to inenoailly hurt him but he comes and hits me like it was a bad thing he is 17and i am 12 now what should i do i went to finsh the convo after he did that and learned my grandma was comeing over to vist our house so what should i do should i tell my mom what happend or should i tell my grandmother