Data can be directly entered in the application on the device.  Most views have "+" button.  Tapping this will provide the options to enter new data based on the view that you have the device in.  For example if you are in the notes view and tap the "+" sign you can create a new not but you won't be able to create a new task etc.

Informant can also be populated by syncing to one of the supported sync services (see the sync section of the getting started article)

Also Informant can be populated with the data from the iOS/Exchange/iCloud syncing that the device can do.  In this configuration the device will share its calendar and reminders data with Informant.

Once populated with data you can begin to take advantage of all that Informant can do for you.  We encourage our customers to try different settings and workflows to find what works best for you.