The "or not" portion of "whether or not" is often unnecessary and can be removed, but sometimes it is essential. Here's the trick: "Whether or not" should be used when one's response is the same, regardless of the outcome. Conversely, "whether" should be used by itself to indicate only that the outcome itself is undetermined.
Think of the expression that children use when playing hide-and-seek: "Ready or not, here I come!" Without the "or not," it would have an entirely different meaning.
"I am not sure whether or not I can attend" / "I am not sure whether I can attend or not" could be shortened to read "I am not sure whether I can attend". In these cases, the "or not" can be safely removed.
"I will be there whether or not I am prepared" / "I will be there whether I am prepared or not" cannot be shortened to read "I will be there whether I am prepared".
Please let me know whether this explanation is understandable. Whether or not I succeed at explaining these concepts, I will keep trying. ; )