When you start a new content, you will be prompted to select a sandbox environment. You will be presented with the following screen:
This environment determines the preinstalled software available to you and your users, as well as any default settings.
Once you select your stack, the page will reload with the corresponding sandbox. If you want to change your stack later on, select Stack in the left sidebar and select the new stack in the Environment section.
You can find instructions for how to install additional software in your environment here.