Can I make my hair thicker?

Ok, so first off, I have dark brown, layered long hair. My account picture is not me. I am a girl. And I’ve never really had a problem with my hair or not liking it, except for the fact that it doesn’t have enough body to it. And the other day, one of my friends told me that my hair doesn’t look good layered because my hair is too thin. How can I fix this? Is there a way to make your hair thicker? I’ve tried using volumizing shampoo, but it doesn’t seem to work. Any ideas?