To agree :
I agree… ⇒ I agree that social media is dangerous.
⇒ I agree that it is difficult to meet people.

To disagree :
I disagree… ⇒ I disagree that social media is for teenagers.
I don’t agree… ⇒ I don’t agree that social media is for teenagers.

CAREFUL! : disagree = don’t agree

To express your opinion :
In my opinion… ⇒ In my opinion, technology is bad for you.
I think that… ⇒ I think that we need to be careful with technology.