Telling the truth may seem simple, but it isn’t. First you have to know the truth. Second, you have to use very precise language when you tell it.
To know the truth you need to have verified the facts you’re reporting, either through direct observation or in some other reliable way. Then you need to report the facts as objectively as possible, neither sugar-coating nor overemphasizing unpleasant details.
If you’re repeating what someone else said, something you read, or what you saw or heard in the media, you have to say so, and cite the source. And you can’t report anything you saw, read or heard from one of those sources as fact.
If you’re stating an opinion, say so. Don’t report opinion as fact. Judgment of any kind, including praise, and especially criticism, isn’t truth. It’s opinion.
You can say what you think, or report what someone else said they think, or what they said they observed, but you need to make it clear that that’s what you’re doing. Otherwise you’re stating opinion or hearsay as fact. It isn’t.