Agreeing
I agree. So do I. Me too.
Me neither. (Agreeing about a negative idea.) I don't either. (Agreeing about a negative idea.) You're right. That's right. Good idea. I think that's a good idea.
Disagreeing
I disagree.
I don't think so.
(No.) That's not right.
Yes, but...
(I'm sorry, but) I don't agree.