How do I stop being so quiet?

In just about every social situation, I am not the one to initiate the conversation. It feels like I lack conviction and confidence in what I have to say, and have often said the wrong thing many times over, which is why I am quiet now. I take lots of time to answer or even respond with a sentence to the person I'm talking to. The way it usually goes is that they will talk about something and I will just intermittently hit them with a "yeah" or a "mhm." It doesn't help that I'm not one to steer a conversation, I kinda just match the vibe of it. What can I do to help me add more substance to conversations with people?