Think about or of ... That is the question…

1) "Think about" is used when you're actively considering something or concentrating on the thought of it.
 
2) On the other hand, "think of" is used when the idea comes to you.
In other contexts, to think of something means you're at least aware of the thing but may not have given it a great deal of consideration. If you think about something this normally implies more focused or extended attention. Some consideration has gone into an idea.
 
3) In most other cases, they are very similar and can be used interchangeably. Especially for actions in the future.
 
4) One thinks of someone concerning a particular event (divorce/death in the family), etc. One thinks about someone in general.