It is best if you download the Zoom app ahead of time. You can get it from the Zoom download page, here.
If you have not joined a Zoom meeting before, there is a test meeting you can join by clicking here.
When it is time for the meeting to start, click on the link for the Zoom meeting. This is most likely something sent to you via email.
When you click the link for the meeting, it should take you to a web browser, which will ask if you want to open the Zoom.us app. Click on Allow, as shown below:
When you click allow, the Zoom app will launch. If there is a password for the meeting, you'll be prompted to enter it. Then you will be connected to the meeting.
For more information such as joining a meeting from another operating system, you can see the instructions from Zoom on how to join meetings here.