To install High Fidelity's Interface, follow the instructions here to download the client installer. The installer comes with everything you need to view and interact with High Fidelity's content and users. You cannot install High Fidelity's Sandbox using this installer, which means you won't be able to create or host your own content.
Want to create and host your own content? Follow the instructions here to download the client and sandbox installer. High Fidelity's Sandbox provides all the tools you need to create, modify, and host your own content.
For more information on the interactions between the two installers, go here.