If you have your music/videos manually managed, you will need to delete the song from the iPod via iTunes. If you have it set to sync, the next time you connect your iPod and iTunes detects you don't have that song in your library anymore, it will remove it from the iPod. To remove an app, hold your finger on an app icon until they all start to wiggle. There will be a - sign appearing next to the non-bundled apps, tap that to remove the unwanted app.