In my opinion,a policeman is the hero in my eyes.

　　In my opinion,a policeman can be regarded as a hero.

　　A policeman is the hero in my eyes.

　　(you don't need both 'In my opinion' and 'in my eyes'. They are repeat as have the same meaning.)

　　In fect,policemen can be seen here ang there.

　　In fact, policemen can be seen in many places where their presences are needed.

　　They catch thieves as well as keep our city safe.

　　They spot and catch criminals and provide safe guard to us to as keep our city safe.

　　(thieves are not the worst people to our society.)

　　In other words,we can't live without them.

　　In other words, we can't enjoy a peaceful life without policemen.

　　If people lost something,a policeman will try to find it.

　　(this is not very true, it depends on what lost as policemen are to busy to look for everything people lost)

　　If people need any help,a policeman will try to give them a hand.

　　(I don't think so. This is a weak point to support your opinion)

　　How great a policeman is!

　　I should learn from them to be a warm-hearted person.

　　I should learn from them and try to be a warm-hearted person.