"If" is used to create a hypothetical scenario with one or more outcome:

"If you leave the house without your umbrella, you might get wet in the rain this afternoon."

"I'll call you if I get home before midnight."

"I'm going to the concert only if it doesn't rain"

"Whether" is generally used to present the same outcome to two or more situations:

"Whether it rains or not, you should take your umbrella with you."

"Whether I get home before midnight or not, I"ll call you either way."

"I'm going to the concert whether it rains or not."