The two most common prepositions used after the verb "think" are "about" and "of." They are very similar, but there is a small difference.
Usually when you think of something, it is a brief moment - just a few seconds. It is also used for opinions. When you think about something, you are considering it for a longer time - like a few minutes or more.
Every time I hear this song, I think of my mother.
(thinking for a few seconds)
What do you think of my new haircut?
I'm thinking about moving to a different city.
I still get angry when I think about all the rude things my sister said to me. (thinking for a few minutes or more)
Common error: Don't use "think to" for "considering."
I'm thinking to do an intensive English course in Canada.
I'm thinking about doing an intensive English course in Canada.
Although you like to think that all of your decisions are carefully controlled and thought out, research tells us that most of our everyday decisions are actually subconscious. There is a reason for this though. Every second our brains are bombarded with over 11 millions individual pieces of data and because there is no way we can consciously sift through all of it our subconscious mind, following certain rules of thumb, helps us out.