There is some difference in meaning here. Let me start with the first part of your question.
To think about it:
You can think about something, for example, what university you will attend or what you will have for lunch. If you are already discussing a subject, you can simply say "think about it".
To think it over:
The verbal phrase "think it over" actually means you will carefully consider something before making a decision. You have all the facts and needed information, but now you want to think it over. If you would replace the word "it" with something else, e.g. "that problem", that would change the position in the sentence: "I will think over that problem" (vs "I will think it over").