Introducing Microsoft To-DoŚnow available in Preview - Office Blogs
To-Do comes from the team behind the Wunderlist app, and delivers a smarter, more personal and intuitive way to help people stay organized and get the most out of every day. Powered by Office 365 integration and an intelligent algorithm, it is the first step on our journey to simplify task management and help you achieve more.
Although To-Do is powered by Office 365, because all to-dos are stored on Exchange Online servers, they will be shown in both To-Do and Outlook.com and Office Outlook Tasks automatically.

All you need is a Microsoft Account. To-Do works on iPhone, Android, Windows and the web. To access via the web version, go to Welcome to Microsoft To-Do and sign in with your Microsoft Account.

To-Do Help & Support as well as FAQs is available here. Click the down arrow to select different platforms or view all at that link.