Introduce something that you've just realized or just remembered this way. "Come to think of it" means something like "Here's something I just thought."
You use "come to think of it in response to something that someone asked you:
A: If you hear of anyone that's hiring, let me know.
B: OK, sure... Actually, come to think of it, I have a friend who's looking for some people.
Or you can use it to add to something that you said yourself:
I haven't been to Spain in a couple of years. You know, come to think of it, I haven't gone anywhere in the last few years.