At this point, your five year old child is like a budding flower. The influence of the outer world is huge at this age. Kids tend to pick up anything and everything around them. No matter what, they will bring home habits that many a time may not be healthy. Thus, letting the kid know what is good and what is bad is certainly something that as a parent you must do at the very outset.
What you need to know
There are habits that kids learn from their peers and find them very cool and smart. Habits like being abusive, back talking or physical aggression like hitting are some common habits that a five year old child learns from the outside world. Right from the very beginning it is important to make it clear that there are certain things that are unacceptable in the home. And you must ensure that the child is not given a single chance to violate the rules. This way the child understands that such behavior doesn't stand a chance, and he must stop those acts and be good.
Outside influence is strong
There might be occasions when the child becomes unruly because the outside influence is more and the child tends to believe what everyone is doing is the right thing. Talk to your kid, and make him understand. Reprimanding is not going to help; instead you can give small punishments to make your point stronger. This way the child will understand and will stop such unacceptable behavior for good.