I am thinking of going to the movie. I am thinking of my girlfriend. I am thinking of what I should wear to the party tonight. I am thinking of the horrible car accident my friend was involved in. Just a few examples. "Thinking of", literally refers to what a person is thinking about, or thinking about doing.

It's true that many people use "think of" the same way they would use "think about." However, "think about" should be used when you are actually thinking about something/someone. "Think of" is different: it means have an opinion about something. For example, "What do you think of Vladimir Putin?" or "What do you think of young children playing video games?" Your answer could be, "I don't think much of it" (which means you don't like it). "Think of" also means "to remind you of something" or to bring up a memory. For example, "This Beach Boys song makes me think of California" or "The smell of this house makes me think of my grandparents."