To create an account find the "Cloud-upload" icon in the right-hand corner of the overview in the web extension. Click the icon and it will show you a popup. There you can choose to either sign up with an existing Google account or email + password.
When you create an account, your current data(tabs, notes, to-dos, groups, categories, etc) will be transferred to the new account. You can then login to your account on this site and access all your data. You will also be able to upgrade your plan and manage your account, including changing your username, email, and password.