If you are playing Pokémon Shining Diamond and Shining Pearl and are hoping to add a Garchomp to your team, then you are in luck. Garchomp is available in the game, but it will take some work.
The first part of the process is tracking down a Gible. Gible can be found in the Rebel Cavern. This cave is located under the bike path on Route 206. When you get to the area, go left and go under the overpass and enter the cave.
There is a secret entrance here which will grant access to the Wayward Cave and you can encounter some rocks blocking your path. You will need the HM Force to overcome them. You can find the HM Force at the culmination of the Lost Tower on Route 209. You must go there and get the HM, and then you will only be able to use it after beating Byron at the Canalave City Gym.
As soon as you manage to move the rock, you may get lucky and find a Gible encounter after it. This is very rare, so it may take a lot of random walking around the area. After that, take Gible to level 24 for his evolution into Gabite, and then up to level 48 to get the evolution into Garchomp. And this is all! There is a lot of work and leveling involved, but Garchomp can be yours if you put the work into it.