As when Ditto transforms it copies the opposing Pokémon's Stats (except HP).
So for example if your opponent was a Lucario with 252 EV's in Attack, 252 EV's in Speed and 4 in HP, equally 280 in HP, 320 in Attack, 170 in Defense, 240 in Special Attack, 170 in Special Defense and 306 in Speed. Ditto would copy the Pokémon exact stats except the HP Stat so it too would also have 320 in Attack, 170 in Defense, 240 in Special Attack, 170 in Special Defense and 306 in Speed.
Because Ditto copies the opposing Pokémon Stats (except HP) it is pointless to EV train Ditto in anything but HP.
Recommended move set for Ditto:
Ditto @ Choice Scarf
Nature: Doesn't matter
EV's: 252 HP