First, you need to get homebrew, preferably CFW, running on your 3DS. *****will guide you through installing the Boot9Strap, a bootrom exploit, and Luma 3DS, a popular custom firmware, on your 3DS.
Afterwards, you can use a GBA emulator to play your GBA roms, or another method you could use is a VC injector, turning it into a "Virtual Console" game.