Think of - You are trying to remember or imagine a specific thing or things: "I am trying to think of a good example". "Can you think of a word beginning with the letter a?"
Think about - There are a few meanings. It means literally, your current thoughts relate to a general subject. It could be used like this: "I am thinking about going on a vacation" or "Could you please think about what we should have for dinner". It is not very contemplative, or specific.
Think (it) over: When you have been given something difficult to consider, and you must give the matter extensive and deep thought. "You have made some very good suggestions, I will have to think them over and get back to you". The preposition suggests that you will turn over a specific problem in your mind to look at it from many angles.