Is this a New 3DS, or an Old 3DS? For a New 3DS, you will need a retail copy of Zelda: OoT (and someone to upload the save to it somehow) or Cubic Ninja, at least for the initial setup. I saw a copy of CN in a local Target for $20, so they do exist, and with the availability of the DS Profile Exploit for New 3DS, I imagine the price of second-hand copies would be falling. Aside from that, there is at least one thread of people offering to install the NVRAM Profile exploit for owners of a New 3DS who do not own either of the required games.
Once you've used Zelda/CN to reach the Gateway launcher, you're able to install an older version of the System Settings app, allowing you to use the DS Profile Exploit (also known as NVRAM Profile Exploit or other similar names), as well as install the provided "Blue Card Fix" which will allow you to use the Blue Card (included with the Gateway) to both reinstall the NVRAM profile as needed, as well as play DS games. The downside is that while you're in sysNand, you lose advanced features such as Super Stable 3D and Wifi SD card management, but you'll still have them while in emuNand (which is a separate copy of the internal system files that runs off the SD card, so you can update it to a newer version without worrying about being locked out of the Gateway.)
If you have an Old 3DS, you can use the Browser exploit, and from there decide whether or not to downgrade to 4.x. Doing so does break the encryption of certain games, causing their save files to be seen as corrupted, however. If you don't have the affected games (or don't mind losing your progress), then there's not much of a downside there. There is more information available in the relevant threads.
I hope I was able to give you a brief overview of the initial process, enough to help make sense of the threads that go into more detail. If you still have questions, feel free to ask.
