Paul writes a charter for Christian thought. We are to center our minds on these exalted things - not pornography, not violence. Those are things of this world, and in this regard, our minds are such that we are like sheep. No other domesticated animal can eat such a wide variety of different vegetables and herbs, seemingly without very much damage. Things that will kill a cow will not kill a sheep. In a similar way, we are capable of taking things into our minds that, seemingly, are not doing any damage at all, but they are definitely not providing the right kind of sustenance for the right kind of thinking.
It becomes our responsibility, then, using the power of God's Spirit with which He anoints us, to choose to think on things that are true, pure, noble, just, lovely, of good report, praiseworthy, and honest. Not much in mass media fits this description. It is our choice, but that is the purpose for which God wants His Holy Spirit to be used.
If we instead think about evil, we will act evilly - we will be evil, because action follows thought. It is as simple as that. Thoughts come from what our minds have been fed, and thus what our minds have been fed will determine how we will act. What we think upon will determine our character and our attitudes.