“Need of” vs. “Need for”

Would you guys please tell me the difference between these words: need for/ need of, think of/ think about? How should I use them in particular cases? Would you be so kind as to give me some examples?
THANKS,

“Need for” and “need of” depend on sentence structure.

After the drought, here is an urgent need for food in Sudan.
Sudan, after the drought, is in urgent need of food.
I have a need for money.
I am in need of money.
(I would use “I need money” instead.)

“Think of” and “think about” can sometimes be used interchangeably.

I think of you.
I think about you.
Think of something that you like to do.
Think about something that you like to do.
Think of a good sentence in which to use “think of.”
I would not use “Think about a good sentence in which to use…”

Hope that helps.