If you want to agree with someone you can say：

　　-That's exactly what I think.

　　-That's just what I was thinking.

　　-In my opinion， office workers are not paid well enough.

　　-That's a good point.

　　-I agree entirely.

　　If you want to disagree with someone：

　　-Quite right， I couldn't agree more.

　　-Yes， I'm all in favour of that.

　　-Well， my opinion for what it's worth……

　　-I can see what you mean， but ……

　　-I don't think so， because……

　　-I don't think it's a good idea……

　　-Maybe， but don't you think……？

　　-That's true， but on the other hand……