I'm just going to assume that when you buy your console it will be on 11.6 Firmware, which means you'll also need a DS/DS-Lite/DSi and a compatible NDS Flashcard (R4i-SDHC etc)
(If it is on anything between 9.0 and 11.3 Firmware it will be even easier to install B9S as you can use Soundhax (see the guide))
You basically need to use an old DS to flash something called NTRBoot onto the Flashcard, then you can use that with the guide above to install B9S onto your 3DS which will allow you to install pirated games and other goodies
If you don't have a 3DS yet, any 3DS you buy will probably be on too high a firmware to hack it without extra hardware.
The good news is, all you need to hack any 3DS is a compatible DS flashcart (this one will work: http://www.nds-card.com/ProShow.asp?ProID=149) http://3ds.guide has the instructions, you can read through it once or twice to familiarize yourself with the process. There are several methods described there, but the one that uses a DS flashcart is called ntrboot (Single System)