Keeping your camera on is preferable in most situations. Do make sure you are dressed appropriately -- as you would if you were in a physical classroom.
If you create a digital background, keep it appropriate and not distracting.
If you are not using your camera, your avatar must be either your initials or your school photograph.
When casual conversation is happening -- for example, at the start of a session -- you can leave your microphone on to participate. Once the meeting or class officially begins, though, you should keep your microphone off. This eliminates background noise (everything from the sound of your typing to your dog barking to traffic passing outside).
Sometimes it can be difficult to interject your comments during a video meeting. The chat window is a great way to participate, especially if you have clarification questions and there isn't a good opportunity to break into the spoken conversation. You can also respond to questions asked in the chat window if you know the answer -- help each other!