"If" has many more uses, "whether" is a very specific use. "Whether or not..." is the most common way to use it. For example, "Whether or not my futbol team wins, I will have fun at the game." or "Whether or not I pass this class, I still learned a lot." (So in these cases, "whether or not" means that in both situations, X will happen)

“If” is for a condition. “Whether” is when there are two possibilities. I don’t know whether he will come Friday or Saturday. (He’s coming. Either Friday or Saturday). I don’t know if he will come Friday or Saturday. (He might not come at all).