You know some things. But most of the time, you barely know anything about that currently discussed topic. And this is perfectly fine. But the biggest problem is to overestimate your own knowledge, because it leads to poor decision making and risk management.
I know that I know nothing — Socrates (who else)
Acknowledge that you sometimes are totally clueless and don't try to hide this fact. It definitely makes you vulnerable, but being overrated over the long term can lead to serious problems and identity conflicts.
Don't get me wrong: I think it's important that you start without knowing what you don't know, because the desire to learn is partly linked to the feeling that you already have some knowledge about a topic, so it is absolutely beneficial for your motivation to be unaware of ones own cluelessness. People prefer to learn something that isn't entirely new, because we hate the feeling of being inexperienced (see e.g. this article discussing this topic). A little bit of self-delusion can be very helpful, but this attitude of "knowing it all" starts to get in your way when you start to work professionally and in teams or any type of collaboration. In this period you have to internalize the concept of conscious incompetence. If you aren't aware of your knowledge gaps you're not able to work collaboratively with people of different fields of expertise. Saying "I don't know" should not be used as an excuse for ones unwillingness to learn new things, but should be seen as a way to acknowledge the competence of your peers and colleagues and as an invitation for others to teach you (although the other one should also be sensitive enough and strengthened in his/her ego to be silent if you don't want to listen to them). I really like the model of four stages of competence and the phrase "unknown unknowns" in this context.
I experienced situations where people overestimated me. It's not a good feeling. Maybe you feel like you just "fake it 'till you make it" or maybe it was relieving for you that nobody recognized your insecurity, but getting praise for something you aren't isn't fulfilling. Pretending to be certain all the time leads to unidimensional thinking and harms creativity, because you start to judge things you don't know anything about as your self-esteem surpasses your competence. And it doesn't add any valuable insights to feel intelligent, too (besides acting like a douchebag).
If you start to think: "I have an educated opinion with valid arguments about any topic you could imagine" (exaggerated, but I definitely met people who thought exactly like that about themselves, although I'm not sure if there was a deep insecurity covert), you should take one step back and think about your self-concept and perception. Reading about the circle of competence and the vast amount of cognitive biases is going to help, it opened my eyes - though it isn't possible to evade them all. Daniel Kahneman's Thinking, Fast and Slow is like reading a book containing "Gotcha!" on every single page, unless you deal with people on a daily basis in which case you'll shout "It is exacty like that".
So don't be afraid to say "I don't know", but be proud of it (not too much, unless you want to be arrogantly ignorant) and ask. And I finalize this article with a quote from the German intro music of Sesame Street:
"Wer nicht fragt bleibt dumm"
If you don't know what that means, just ask.