"whether" has a very narrow and specific meaning of "if something is true or not", so it is good to use it if you are specifically focusing on that, especially something factual. "If" includes the meaning of "whether" as well as other uses like conveying an implied wish, and used much more broadly than "whether". Examples: * Tell me whether you did this. ("if" can replace "whether".) * Tell me whether or not you did this. (more strong. "if" will make it sound bad) * I don't know if hi is ill. ("whether" can replace "if") * If I knew what to do ... ("whether" is not acceptable) * If and only if ... (same) * If I was a millionaire... (same)

In most cases they are interchangeable. Which means either one can be used. Usually after something like; about, you would use whether.