We'll be programming the language Scheme. In particular, we're using Guile, which is a particular Scheme interpreter. The preferred approach for doing this is to use Repl.it. You don't have to install anything locally on your own computer to do so. Most importantly, Repl.it allows you to pair program remotely simultaneously with others, and also share your code with us in real-time so that we can help you diagnose problems as you are working on them.
However, if you find that you wish to also install Guile locally on your computer, you likely can. Here are instructions on how to do so. I want to again emphasize that Repl.it is the primary approach that we're using, and it will be much more difficult for us to support you remotely if you only install it locally. But I also understand that you may experience connection issues which may push you towards a local install.
Repl.it has Guile version 2.2. (Technically, as I'm writing this it is version 2.2.3, but the last digit doesn't really matter; it represents very minor updates.) So you'll need to make sure that your own computer has a 2.2.x version. You might stumble across versions 2.0.x or 3.0.x, but they are incompatible in some ways that can matter.
Guile on Macs is supported via Homebrew. If you haven't already installed Homebrew on your Mac, see the instructions on the Homebrew website. There's a long one-line command that it instructs you to copy and paste into a terminal window. Once you have installed Homebrew, you should then be able to install Guile with the command
brew install guile
This will install Guile version 2.2.7 (at the time I'm writing this), which is great.
Guile on Windows is supported via Windows Subsystem for Linux (WSL). If you haven't already installed WSL on your computer, see the instructions at the Microsoft WSL installation site. If you are doing this for the first time, select Ubuntu 18.04 LTS. Once it has been installed, start WSL using these launch instructions. This will give you a terminal window. To install guile within, follow the instructions in the Ubuntu Linux section below.
The following instructions are for Ubuntu; if you are using an alternative Linux distribution, you'll need to figure out how to adapt accordingly. You'll first need to figure out which version of Ubuntu you're using. In a terminal window, enter
lsb_release -a
... and it will tell you what version of Ubuntu you have. Then enter the following commands, depending on version.
sudo apt update && sudo apt upgrade
sudo apt install guile-2.2
sudo add-apt-repository ppa:dank/guile-2.2
sudo apt update && sudo apt upgrade
sudo apt install guile-2.2