To show agreement:  

• I agree with you
• I go along with you
• I subscribe to the idea that...
  
• I am in the agreement of this point
• I see what you mean
• I share your opinion
• I see your point
• I suppose that it's true, actually
• You might be right there
• That's a good point
• Well, I can't argue with that
• I suppose, you've got a point there
• I am convinced that

Words to express your opinion: believe, think, argue, feel, reckon, opinion, view, convinced, etc