The difference between about and if...

Can anyone tell me when I should use "about" and "if"? I have heard people saying "we are thinking about you" and " we are thinking of you"... I am confused... what's correct?

ohhhhhhhhh I did a mistake... I wrote "if" instead of "of" :((

"We are thinking of you." has the additional meaning that action is being taken based on that thought.

Example:

Let's say some people are eating together somewhere, but Person A doesn't have anything.

Person A: Can I have something to eat, too? (feeling left out)
Person B: I thought of you. *gives food*

Thank you Guest :)