With a name like Sonic Pi, you might think that The Live Coding Music Synth for Everyone (their words, not mine) forgot about everyone not using the Raspberry Pi. However, that's where the name Sonic Pi might be a bit misleading — it was developed in collaboration with the Raspberry Pi Foundation, so comes pre-installed on Raspbian images for the Raspberry Pi.
Sonic Pi is also available for Windows, Macintosh, and Linux. So how would you get started with Sonic Pi on the computer you already have? I thought you would never ask.
Raspberry Pi loves Sonic Pi so much that it comes pre-installed on the current Raspbian images for the Raspberry Pi. All you need to do is download the current Desktop version of Raspbian from the Raspberry Pi Foundation. You can also download the current version of NOOBS and use that to install Raspbian.
If you're not sure how to write the downloaded image to an SD/microSD card, their website has instructions you may follow.
Windows, Macintosh, or Linux
For the remaining three operating systems, proceed to the Sonic Pi website and download the installer for your operating system. They also have installation directions available.
For Windows, they also have a version that does not need to be installed, but may be simply copied to and executed from a removable USB drive. This would be a good option if you need to try Sonic Pi on a computer you do not own.