Two minutes of Googling would tell you.
You can install NTR CFW which will let you stream wirelessly, but the framerate is rather low and highly dependent on local wifi traffic. This will only work for software running in 3DS mode and not for DS games or GBA games (running on AGB_FIRM).
You can get a hardware capture card, but those are rare and expensive as the original producer stopped making them.
Or you can just run your stuff on Citra in Windows.