Things that are automatically collected (don't need fields added):
User - the logged in user who added the entry
Date of entry - the date the entry was made
Date of last update - the date the entry was last updated
Adding a field:
Go to the "Fields" tab when viewing your database
Under "Create a new field" choose the field type from the drop down menu.
Every field will ask for a Name and Description. These are arbitrary, make them something short you will associate with the data being collected (and the name and description can be the same thing).
Depending on the field type there may be other settings. Please see the link above to the MoodleDocs page for more information on each field setting.