To login to Microsoft Teams using a laptop or chrome book please browse to teams.microsoft.com
To join a video please press the Calendar icon on the left hand side of the screen and then simply click join
To connect to a Teams meeting using the mobile app please use the Microsoft Teams App (install from here)
Choose the calendar icon and then click join next to the meeting you want to attend.
Please see the video below for a quick introduction on how to join the meeting.