When you're discussing topics they are some expressions you can use to sound more natural in English, for example to ask somebody's opinion "Helen, what do you think about this?", to state your opinion "The way I see it..." or to interrupt if the other speakers don't let you talk "Can I add something here?".
Besides, it's also necesary to agree or disagree with others in a conversation. So, you can use "I'm with you on this" to agree, or "I'm not sure about that" to disagree.
These are only some examples, but there are many others you can use.
Here you have a document with a list of useful expressions. I recommend you print it and use it when discussig topics. It's good practice for the exam!