If you've enabled SpotPass on your 3DS (and if you haven't done so, or don't know what it is, you can find out more here), you'll likely have noticed your console's notification LED blinking in an appealing blue at you, with a message letting you know that a new system update's available. All you have to do is download the system update, and the wonders of the eShop will be yours.
To receive the update, you'll need to have set your 3DS up to talk to your wireless router - if you've received the SpotPass message, then you're good to go. If not, take a look at our guide to setting up the 3DS's internet, and meet us back here when you're done.
With your internet properly set up, all you have to do is scroll across to System Settings, touch "Open", touch "Other Settings", and then press the right arrow on the Touch Screen until you come to "System Update". Touch the icon, and you'll be asked if you wanted to connect to the internet to download the update (in case you'd somehow managed to randomly scroll across to System Update and pressed it without actually intending to update your system), at which point you press OK, sit back, relax, and watch as your 3DS downloads the latest update.
The update brings with it a number of new additions, including several improvements to the 3DS's performance, the ability to use buttons as well as the touch screen to confirm certain menu options, and, the biggies, the 3DS eShop, and two free games - Pokedex 3D and Excite Bike 3D. Once you've downloaded it, and accessed the eShop, make sure you check them out - they're well worth a gander, especially considering they're free.