Effect, generally used as a noun, infers the result, cause or agent. Thus, you’d use effect when you’re talking about an outcome or something caused by something else. If you were discussing new management you may write: "It’s interesting to see the effects the new rules produce."
The word “affect” means to influence. Typically, you should use affect as a verb: "His speech affected me on an emotional level." Use affect as a noun when describing facial expressions: "Eddie had a sad affect."
There is an instance where you use effect as a verb -- when you’re writing about something produced or change brought about. If you want to talk about a new boss making positive changes in your work environment you’d write: "Dave, the new manager, effected some positive changes in my office." Using effect in this way shows the changes brought about by the new manager."
Use affect when you’re discussing something or someone’s influence — rather than causing the change. If you’re writing a psychology paper, you generally use affect as a noun. Psychologists use “affect” instead of “effect” as a noun because they can’t positively know how patients feel — they can only see how they appear.