In a cause and effect essay, you are answering the questions "why did this happen?" and "what was the result of it happening?" In an essay about the effect of bad friends, you already have the cause: bad friends. So all you need to do, after explaining what "bad friends" are, is to write about what the effects of that might be, whether cutting class, being talked into drinking or taking drugs, or whatever. It depends on why your friends are "bad" as to what kind of trouble you might get into, or what other effects there might be.
I hope this helps get you started!