Think of something at which you are an expert. Really truly an expert. Try to think about how you feel when you hear a layman talking about that subject. Maybe it's fishing, maybe it's science, maybe it's parenting. It could be the statistics for your favorite football player, something about cars, maybe you are an expert on a band you love.
Think about how ignorant people sound when they talk about your field of expertise.
Now try to remember that, outside your field of expertise, you are just as ignorant. You are just as full of misinformation and misconception about the entirety of the rest of the world, as those people who misunderstand your particular area of expertise.
Think about that every time you have an opinion, every time you think your voice deserves to be heard, and every time that your beliefs are challenged by something you have heard. Remember that listening to the news, reading a blog, and talking to your friends is not a substitute for reading primary research. No one will ever be able to consume all the information required to become an expert on everything, rather as you learn more about a subject you often find out how little you truly know or understand about the world.
Little is more damaging to a critical mind than undeserved self-confidence in one's own ability, or the untouchable elevation of unsubstantiated belief.