To install the app: When you click the meeting link, it should open a web page where you can download and install the app. This only takes a few moments, but you will have to click "allow" or "run" or something along those lines to make it go. If prompted, also allow it access to your audio/video devices.
To troubleshoot audio: Click on the ellipses (...) in the bottom right corner of the meeting window and select "Meeting Options." Then click on "Audio Device" in the list on the left. You can then select and test the various speakers and microphones connected to your computer.
To share your screen or files: Make sure you are listed as a "Presenter" in the participant list. If not, ask a Presenter to make you one.