To express your opinions, you can use an expression like I think … or I guess …

I think you are making a mistake.
I think you ought to try again.
I think that was a stupid thing to do.
I guess she has a problem.
I guess you are mistaken.

The expressions I feel … and I reckon …. can also be used to express your opinions.

I feel she is right.
I reckon that was a mistake.
I really feel that you are trying to hide something from us.

If you do not think that something is true, use the expressions I don’t think… or I don’t believe…

I don’t think you are telling the truth.
I don’t think he is coming.
I don’t think that we have met before.
I don’t think you’ve met my parents.
I don’t believe she’s at home.

If you want to ask people what they think about something, use the structure ‘What do you think of…?’

‘What do you think of my hair color?’
‘What do you think of my new project?
‘What do you think of his girlfriend?’

To ask someone if they think something is a good idea, use the structure ‘What do you think about…?’ or ‘How about…?’

What do you think about applying for that job? OR How about applying for that job?
What do you think about hiring a domestic help?
What do you think about going on a vacation?
What do you think about buying a new car?
What do you think about moving to a bigger home?