How do you get the defog TM?

Answer 1

Defog doesn't exist as a TM, nevertheless it does exist as a HM in Diamond, Pearl and Platinum. Getting the HM depends on the version of your game. In Pokemon Diamond and Pearl, you get the HM from an NPC when you enter the Great Marsh. You will need the Fen Badge from the Pastoria City's Gym in order to use it outside of combat. In Pokemon Platinum, you will find the HM deep inside Solaceon Ruins. You will need the Relic Badge from the Hearthome City's Gym to use it outside of combat.

Answer 2

In pokemon diamond you have to go to the great marsh in pastoria city then talk to the ace trainer on the right right by the gate and you will receive it.

Hope this helped :D