apt-get is the command–line tool for working with APT (Advanced Package Tool) software packages. It is the preferred way of installing packages on the Linux distributions Code Ocean uses (Ubuntu and Debian).
Apt-get is natively accessible in all base images when configuring your computational environment.
Typically, you will know the name of the package you need from the homepage or documentation of the code, or from higher–level library you're using. You may need to search to find the exact name. Feel free to reach out via live chat or email if you need help finding a package.