The easiest way of getting the software is to download the torrent file for the latest official SD card image from here:
It's a 9GB image file (with plenty of soundfonts) compiled for Raspberry Pi 3. (You need BitTorrent software to use the torrent file to actually download the large image file)
If you prefer a smaller image (Gorgonilla) or need support for Raspberry Pi 2, then you can download the older Gorgona image torrent file:
These SDcard images include all the Zynthian software, properly configured for the standard hardware -- ready to use. Plug and play!
After downloading the SD-image, you have to write it to a SDcard. You need a minimum size of 16GB SDcard for the full Gorgona image or 8GB for the Gorgonilla lite image.
Use Etcher https://etcher.io/ (free software works on Mac, Windows or Linux) to write the image onto the SDcard.
Or do it manually: There is a really good manual for this at this link.
If you prefer, you can build your own SD image following these instructions: