Based on my experience if you want to start modding you would need a CFW that has layeredFS patching. (Luma3DS CFW) If you already have that, you would need to use GodMode9 to dump the romFS and the exeFS from either the cartridge or the .CIA/.3DS file (this is obtained by dumping the cartridge to .CIA or .3DS). Then (if you are using a Pokemon game) you would need to download pk3DS onto your PC. Insert your 3DS sd card into your PC, then in pk3DS you need to select the romFS folder. It should load it and recognize your game. You can now edit a whole bunch of things. For more info, look for other threads and look online (Google is your friend ( ͡° ͜ʖ ͡°)) Sorry for the possible confusing tutorial thing. Oh and... go here to get CFW:
https://3ds.guide