I followed the tutorial by Marcus Hellberg on building web apps in java https://www.youtube.com/watch?v=qUBt8k4pQgQ and i want to implement a login page to access the to do app. Can someone point me to a tutorial or documentation on how to do this? Thank you!
There are several ways to implement a login / logout mechanism.
You can for example implement something like that yourself really easy, if it shall be pure Vaadin by using the interface ViewChangeListener and adding it to your Navigator instance. Inside the ViewChangeListener you can access which view is going to be loaded and restrict that to your needs (like being logged in or having several, self defined access rights and so on).
Here is the documentation about the ViewChangeListener: https://vaadin.com/docs/v8/framework/articles/AccessControlForViews.html
and some additional information about reading / writing session values and handling logout:
https://vaadin.com/docs/v8/framework/articles/SettingAndReadingSessionAttributes.html
https://vaadin.com/docs/v8/framework/articles/HandlingLogout.html
The form for the user credentials is nothing special, you just have to check the validity of the credentials and store the information about the logged in user somewhere (for example in the session). When logging out they should of course be removed from the closing session.
Hope this helps as a start.