Mute yourself when you aren't speaking
Even quiet background noises are distracting. If you use the mic built into a laptop, avoid typing when un-muted. Speak at a normal volume.
Notification noises are very loud to other participants in on a call. Set any alerts to silent and find a quiet place to video call.
It is much easier to hear each other an will help prevent mic feedback. Many headphones have built in mics that are great for video calls.
Don't talk over each other
Lag can make asking questions out loud difficult. Instead, try raising your hand or send a text chat. Use "thumbs-up/down" when able.
Audio or video freezing?
You may have a bad internet connection. Try turning off your video, or using an ethernet connection, especially with bigger groups calls.