To sync tasks and notes across devices you simply need to be on the same account on both devices.
To sync events you first need to add the same calendar accounts (Google, Yahoo, ...) under iOS Settings app > Accounts & Passwords. Then both devices will be synced with the same events. Then you need to make sure you associate your new events with your online calendar account (Google, Yahoo,...) so 24me can sync those events. You can see which calendar is chosen for each event by tapping on the event and going to the Calendars tab. There you can see all your calendars and tap on the one you want to use. You can also go to Settings > Visible Calendar to hide unneeded calendars and change the calendar details that will be automatically be associated with any new event you add in 24me.