Believers Church, Chesapeake Virginia

Our culture says it doesn't matter what you do; if it feels good, it is permissible. Christians seem confused. Some say certain things are okay, like having a drink, but not other things, like having a smoke. Some can watch R-rated movies and don't think much of it and for others they view it as sin. How do we know what is sin and what is not? Which prohibitions are cultural and which are biblical?