Thanks for posting this in the Global Canvas Community!
For this question, you might need to reach out to your schools IT helpdesk. Many schools will provide students with a free version of Microsoft Office when they enroll in classes. If that it the case for you, they would be able to give you directions on how to download and start using it.
If you are just trying to download and pay for Microsoft Office, you can go HERE to Microsoft's page to purchase the program.
Hopefully this helps!