Keiko: To first be a good voice actor, you must know the basics of theatre and performing arts. The key to a good voice actor is two concepts: Emotion and Projecting/Portraying. To understand emotion, one must embrace ones own humanity.
Characters are no different from human interaction and emotions, even villains have emotions. For example when a character is happy, one needs to give that genuine feeling of happiness, and vice versa for all the other emotions.
Now how one projects or portrays these emotions has to do so adequately, succinctly, and as close to accurate as possible. By doing so you convey emotions not only to the target of which you are referring to, but in front of the potential audience that is sitting to listen to it. The more genuine the communication, the more believable the experience is.
In short: Be more human. :P
As for different voices: Try practicing your vocal ranges. sometimes twisting your tongue a certain way or positioning it in a certain way gives a new vocal range and accentuation shift.