I am thinking in someone/ something
I am thinking in work (I mean the intention to work as a teacher for example)
I am thinking about...
I am thinking of...
I am thinking about working as a teacher.
I am thinking of my sister who has a difficult exam today.
The only time I can imagine using "think in" would be, for example "I'm thinking in Spanish". That means the thoughts that are going through my head are in the Spanish language, instead of the English language.
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.