Prejudice is the child of ignorance.

A prejudice is what fills the space where knowledge should be. Where you have no actual experience of a group of people, the mind does not leave the slot empty; it borrows a ready-made picture from whatever was lying around, and that picture then feels like something you worked out yourself. This is why prejudice is strongest at a distance and weakens on contact.

Pick a group you have opinions about and few actual dealings with. Go and find the direct experience: read what they write about themselves, or better, spend real time with one of them. Let the specific person disturb the general picture.