In this websites you can find interesting and useful expressions to express personal opinion.
List of expressions
How to express opinion (with explanations and examples)
Agreeing and disagreeing with an opinion
Useful phrases to express opinion (with Spanish translation)

this video to learn more things about giving opinions. It is a good way
to practise your listening skills too, as it is a clear English and you
can read the trascription on the subtitles if you need it.