Thank you
Is it necessary to use "think about" for things
and "think of" for people?
Not necessarily, no. Those were just the two examples I randomly chose.
We are, however, more likely to use "thinking about" for a lot of things.
I'm thinking about going to China.
I'm thinking about changing my job.
I'm thinking about John's problem with his car.
I'm thinking about that programme I watched on TV last night.
I'm thinking about a lot of things.
I was thinking about Sarah yesterday and trying to work out how to help her with her assignment.
There is one instance where "thinking of" is more likely - when we are expressing our sympathy or condolences to someone. Quite specifically, when someone dies we might send a card or a message to a relative of the deceased and might use "I am thinking of you". It does of course mean "I am thinking about you" but it conveys are more sympathetic emotion.