Located within Crooner's Café in Mauville City, you will find this person who is willing to teach Keldeo and Meloetta their secret moves of Secret Sword and Relic Song respectively. When Keldeo learns this move, its form is automatically changed whereas Meloetta changes form when the move is used.
Basically, you need to go to Mauville City and locate Crooner's Cafe. Just Fly to Mauville City and head directly south of the miniature Centrico Plaza-like tower. From there, make the next left, then head north and you should see a burgundy (brown) colored building; this is the cafe. Talk to the balding guy and he'll be glad to assist you.
If my directions are unclear, just watch here.
Hopefully I've Helped!