Once you click on that link, you'll be in the green. If this is your first time in Riverside, you'll need to request permissions for your microphone and camera simply press requests, camera permissions. This will prompt a pop-up asking if Riverside can access your microphone and camera just select allow, make sure to give Riverside access to your microphone camera and even your screen.
If you plan on doing any screen sharing, now that Riverside has access to your microphone and camera, you can set up your audio and video settings before joining the session. If you're using an external microphone or camera, make sure to select the right ones in the down. Once your settings are dialed in, you can enter your name and select whether you're wearing headphones or not.