In order to evolve your starters, you have to:
• Defeat Darkrai at the end of the postgame story
• Do three or four Jobs
• Manaphy will come join your team a few days later and tell you about the Marine Resort
• As you walk through Treasure Town, the Ursaring bros will tell you that something "feels different" about Luminous Spring. It has something to do with time and space being set right once again.
• You are now able to evolve your starters at Luminous Spring! However, some starters need to meet specific requirements.
Pokémon like Riolu will need 1 1/2 IQ stars and a Sun Ribbon to evolve.
Pokémon like Eevee, Skitty and Vulpix will need the appropriate Evolution Stone.
Most starters, like Meowth, Bulbasaur, Cyndaquil, and Chimchar, will simply have to be the right level.
Depending on where you are in the story, this will either be easier or harder for you to accomplish. Good luck!