Know = be familiar with: ‘I know Frankfurt very well.’ (ii) be aware of the truth of something:  ‘I know that he is lying.’
Get to know = become familiar or acquainted with: ‘Once you get to know her, I’m sure you’ll like her.’ ‘We got to know each other very well during the week we spent together.’
Find out = get information about something that you want to know: ‘We need to find out why these cars have stopped selling.’