You may have received an invitation to join your community by email. This email has the subject "Finish Registration for Instructor...". The first step in completing registration is setting your account password so click the "Set an Account Password" button to set your account password, finish completing registration then click the red Join a Community button and follow the steps on the screen to purchase access to your community!
If your professor provides you with the Community Lookup Key then you won't need the invitation email! Either log in or create an account then follow the same steps above to join your community.
If you were invited to join your community with one email and you end up creating a separate account using a different email address, you can still join your community! Simply go to the invitation email and find the Community Lookup Key. After logging in, go to the Join a Community page, paste the lookup key into the search field then follow the steps on the screen to join your community!
Not sure where to find your lookup key? Check out this article: Where can I find my community lookup key?