Go to classroom.google.com.
Click on the class that you need to join.
In the header at the top of the page, look for "Meet link" followed by a web address. Click on that address.
If your teacher has opened the session, you'll be able to join.
Some common reasons that Google Meet might be "jumpy" (lag) or "kick you out" (disconnect) include:
A slow Internet connection. Go to speedtest.net and run a test. You'll need at least 2Mbps download and 1Mbps upload for a Google Meet session with 10 people -- including the teacher. The higher the numbers the better, but much more than 4 or 5 Mbps per student in the home probably isn't needed.
A congested home network. For example, if the speed test listed above showed 15Mbps download, but there are 5 students in Google Meet at the same time and someone in the house is watching Netflix, the network is suffering a sort of "traffic jam." Cut back on any unnecessary things, such as Netflix, YouTube, Spotify, and other streaming or gaming services. If this isn't enough, you may have to call your Internet service provider and ask about increasing your connection.
Too many tabs. For the chromebooks from the school, students shouldn't have more than two other tabs open when in Google Meet. Those tabs shouldn't be streaming music or video, shouldn't be video games, etc. Something like Google Classroom or Google Slides shouldn't be a problem.
Plug in to power. When a chromebook isn't running on battery power, it can "guzzle" power instead of "sip" it. So it can go slightly faster without worrying about running down the battery as quickly.
Weak wifi signal. Wifi signals get weaker the further you are from the signal source. Work with your chromebook near to the router in your home, if possible. The fewer walls in the way, the better. For this reason, it is often helpful to place the router on a counter or table near the middle of your home, rather than in a far corner and out of view.