I don't think the 'just' is a necessary part of the expression, although it's often appropriate in context. 'Shoot' means 'OK, I'm ready, go ahead and say it':
'I didn't quite catch that number. I'll just get a pencil and paper; hold on.... OK, shoot'
You can also say 'Fire away', with the same meaning.
An example of 'just shoot': If you were playing a game of pool/billiards with a friend and he was delaying the game by talking too much rather than playing you could say: 'OK! Quit jabbering and just shoot!' ;-)