When you just want to listen to/watch the talks, opening Zoom at the address
above (and sent by e-mail) is all.
When you present a talk yourself, you should open your presentation on your notebook or PC and, once your slot is announced, click "Share Screen". A new window will open where you can select the application showing your presentation. This will then be seen by all other participants (together with a small window showing yourself if your camera respectively video setting is active). You may use the mouse as a pointer. Please don't forget to turn on your micro!