Ditto can be acquired by getting 10,000 tix from the arcade. A man inside the arcade will give you 5,000 tix for 100 Robux.
The arcade is in Anthian city inside the golden pokeball.
Ditto's only move is transform, which will make him appear as your opponent's Pokemon, and give him all your opponent's move.
It can be breed with any pokemon in the daycare except for legendaries and mythicals as well as itself. (The only exception to the legendaires and mythicals can't breed rule is Manaphy, when bred with Ditto you can get a Phione egg.)