How do you breed riolu Pokemon diamond?

To breed a Riolu, you need to put a Lucario/Riolu with a Pokemon of 1. The opposite gender, and 2. Sharing an Egg Group with Lucario/Riolu into the Daycare Center.

(Those Egg Groups being Ground and Human Shape)

OR

Put a female Riolu / Lucario with a Ditto in the Daycare Center

Following either step, an egg will be produced. Walking/Biking with this egg in your party will hatch it into Riolu. (6656 Steps, which can be halved by bringing a Pokemon with either Flame Body or Magma Armor as it's ability)