"If" is used for Condition/Hypothetical Situation.
"Whether" is used for Choice/Alternatives.

Eg: If you study diligently, you will score high. (Condition)
I don't know whether I will go to the dance

Here's an add-on to Charan's point:

'whether or not' is incorrect on the GMAT. The term 'whether' is inclusive of both cases. i.e.
Let me know whether you are coming to the party or not. <incorrect>
Let me know whether or not you are coming to the party. <incorrect>
Let me know whether you are coming to the party. <correct>

Sometimes, the use of just 'whether' does not bring out the intended meaning. You will 'feel' that an 'or not' is required. In these cases, there is a better way to phrase the sentence. For instance,
The building is to be torn down whether or not the residents move out. <incorrect>
The building is to be torn down whether the residents move out. <incorrect>
The building is to be torn down irrespective of whether the residents move out. <correct>
The building is to be torn down regardless of whether the residents move out. <correct>

Hope this helps!