You have to go to the Move Tutor in the Survival Area. You'll need 6 Red Shards and 2 Yellow Shards.
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Garchomp can only learn Outrage if you breed it.
Go to the battle frontier
By breeding.
Breeding outrahe is an egg move
Salamence can't naturally learn Outrage in Diamond, Pearl and Platinum and it can't learn it through breeding either however in Platinum you can teach it Outrage by going to the Move Tutor in the Survival Area located in Route 226, you need Rock Climb to get up to the house as well as 6 Red Scales and 2 Yellow Scales in your bag in order for the Tutor to teach Salamence the Outrage move.