The easiest way of getting the software is downloading the last official SD image from here:
This SD-image includes 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 flash 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: