Here are some Italian phrases for expressing your opinion.
By the way, be careful with ‘I agree / I don’t agree’, which are verbs in English but prepositional or adverbial phrases (or something like that) in Italian.
It’s as if you are saying ‘I am in agreement’ rather than ‘I agree.’ Italians speaking English tend to say ‘I am agree’ and ‘I am not agree’ for this reason…
A mio parere – In my opinion…
Io penso che – I think that…
Sono d’accordo. – I agree.
Non sono d’accordo. – I don’t agree. / I disagree.
Credo che… – I believe that…
Sono convinto/a che… – I am convinced that…
Trovo che… – I find that…
Per me… – For me…
Secondo me… – In my opinion…