A good Nintendo DS emulator is NO$GBA, which can play both DS and GBA games. A GBA Link Cable feature is also available. Google it.
When you have that downloaded, just go to a ROM site like ROMulation, Coolrom, Doperoms, or EmuParadise and download a game you want.
Note that you need a really good Graphics Card in order to play a DS game without lag. Not many people can use these emulators well without buying a fairly expensive computer.