If you don't have a Microsoft account already and would like to accept your invite to a Sharepoint site, click on the option to create a new account.
Enter you email address and click next.
Enter a rockin' good password and hit next.
Enter your Region and date of birth and click next.
Enter the verification code you receive via email and choose next.
Solve the puzzle to confirm you're not a robot.
If you pass your puzzle test, you should be good to go, minus potentially setting up your 2-factor authentication. You can read more about that process in this tech article.