What to say during a conversation, awkwardness!?

During a conversation, when it's my turn to say what I 'Think', I get lost on what to say when the other person said what they need to say so I just end up saying 'oh' every time and that completely kills the conversation because I feel like I can say so many things after the other person talks but I don't and it makes it awkward, I feel like I'm being rushed to talk so I feel like really stupid. Any tips on how to feel less 'weird' during conversations? Thank you! It's okay to not be talkative, right? It's not rude? :O