How do you download Gamestop Nintendo 3DS games?

I like my Nintendo 3DS, but it won't play gamecards, and I have neither the time nor the money for it to be repaired. So I've been downloading games onto it via Nintendo E-shop, and then I noticed that It was cheaper to go on GameStop.com and get downloads there, but I am still completely new to downloading games onto my system, so I need advice before I pay for anything. So do you get a code and if so how to I enter it so I can get the game.