Installing Scheme on your own computer

DrScheme has versions for OS X, Windows, and Linux. Download it as part of the PLT Scheme system.

Installing C on your own computer

OS X: Install the Xcode developer tools. Choose the appropriate version depending on which version of OS X you're running. You'll then be able to access the gcc compiler from a terminal.
Linux: Any self-respecting Linux distribution already has the gcc compiler installed.
Windows: Install Cygwin, which is an environment that gives you a prompt and software that looks and feels just like Linux. On the Cygwin page, click "Install Cygwin now" on the top right. This points you towards an installation executable. Run it, and choose all defaults for the first few screens. You'll then need to arbitrarily choose a mirror site from which to download. Finally, on the "select packages" screen, expand the "Devel" category, scroll down, and select the package "gcc". Feel free to skim the list of packages to see if you see any other favorite open source tools you'd like to have around. In particular, look at the "Editors" category, as you might find an editor there you wish to use. When done, click the Next button at the bottom. Once installed, you'll get a shortcut on your Start Menu and on your Desktop that will give you a UNIX-like prompt. Note that your home directory within Cygwin is actually the Windows folder C:\cygwin\home\username. You'll need to know where this is if you want to find your files using traditional Windows navigation, such as when you want to submit them.

SWI-Prolog has versions for OS X, Windows, and Linux. Click on "Download" on the left, then choose "Stable release." Choose the appropriate version.