For a quick and easy video on joining a Teams meeting please see the Joining and Participating in a Teams Meeting from Microsoft otherwise you can follow any of the methods outlined below:
From your calendar
Click Meetings on the left side of the app and you'll see a list of all your meetings for the week. Find the meeting you want, and then click Join.
From chat
If the meeting has already begun, it appears in your recent chat list. Select the meeting in your chat list and then click Join from the chat header.
From a notification
If you're busy in Teams, you'll receive a notification that you've been invited to a meeting, and you can click Join there.
From a channel
If a meeting is happening in a channel, you’ll see an invitation to join, relevant content, and who’s in the meeting. If the meeting is scrolled out of view in the conversation, you'll get a notification in the channel. Just click Join to choose your settings and join the meeting.
From Outlook
You can also join a meeting from a calendar invite in Outlook. If you click the link in your email invite, you'll be directed to Teams and can join your meeting from there.