One of the important things to do when you join a Microsoft Teams meeting is make sure your video and audio options are set properly based on your current environment. It used to be that you’d have to join the meeting first, and then look for the options to fix everything (like muting yourself or turning on your video). Now there’s a new pre-meeting joining experience to let you set those before you enter the meeting. Here’s how it works…
When you join a Microsoft Teams meeting using your desktop client, you’ll see the following panel before you actually enter the call:

This allows you to turn your video on or off, set any background images, pick your audio preference, and put yourself on/take yourself off mute. Once you have everything to your liking, click Join now.
It seems that your settings carry over from meeting to meeting (in my experience), so most of the time you can just click the Join now button to get into the meeting. But if you need to change over to a different audio source or don’t want people to hear your background noise, you can set all that up before making your grand entrance…