Microsoft Teams is built into Office365. It is a video conferencing and communication tool.
To access it, log onto Office365 (either through email or the login button on the school webpage or login.microsoftonline.com).
Click on the three dots (top left of the screen) and select Microsoft Teams:

You can also download the Teams app for Windows, Apple Mac, iOS, PadOS, Android.
NOTE: On Apple Macs, iPhones and iPads; Teams does not work with Safari. Either install the App or use another browser (eg Chrome)
You'll find the App at the bottom left of the screen:

Click this to download the App. Run the installer and follow the onscreen instructions. The App offers more features, such as the ability to see more people on screen when video calling.
The first time you run the App, you will be asked to log in to Office365.
The main screen you will see once you have logged in is the Teams screen. Here any Teams groups you are a member of are displayed:

On the left-hand side is the menu.
Activity is anything that has happened in Teams that affects you or the Teams your are a member off
Chat is the messenger app. It works the same as your phone message app, just only texts people in school (within Teams)
Teams is the main screen (shown above)
Assignments relates to assignments and tasks that you would set a class (if using in lessons for remote learning)
Calendar your Outlook calendar
Calls again, just like your phone, allows you to direct call other Teams users. Either video or audio calls (and obviously only if they're online)
Files allows you to access files you have uploaded to Files you've recently access in Office365*, Teams, Your OneDrive and you can add other cloud storage accounts too (such as Dropbox).
*So if you've been working on a file in Word, it will appear in Recents so you can easily access it from, or upload it to, Teams.
... is a menu containing additional Apps (such as Office365 ones) that link directly into Teams.