Download the Teams app on your PC. https://teams.microsoft.com/uswe-01/downloads
Once added to a Team you will receive an email titled "You've been added to a team in Microsoft Teams" Click the link “Open Microsoft Teams” in the email and it will open the Teams app.
It will ask you to sign in, use your email address account as the username, this will be the email address you received the invite too, you may need to sign up to teams if you have not done so yet, to do this click Sign up for free at the bottom.
Once signed in your will see all the teams you are assigned to by clicking on the Teams icon on the left hand menu pane.
Click into the team to post messages or join channel meetings.