Prepositional use is the measure of fluency in English.
"I think of you" You, specifically, are the person in my mind.
"I think about you" I think about your personality or characteristics.
Maybe this is more concrete: "I think of a number between 1 and 10" It will be a specific number, say 2.
"I think about a number between 1 and 10" It could be any of them, 2 through 9
Admittedly, they can sometimes be very close in meaning.
For example: "Whenever I smell apple pie I think of/about my Mom" But there is a difference: do you feel it?