Default [Q] [SGH-I317] How to go back? Unroot and Reinstall Stock

Hello, One of my buddies rooted and installed a rom on my phone. Its cool and all but I would rather have all my Note 2 stuff back to be honest.
Now my buddy is telling me " Just install a new rom " every time I ask him to help me to remove all he did...

After long search I found nothing to help me with my problem.. I really want to go back to my old Note 2.

Please help me