1) yes, without you will damage your Arduino.2) It top switching 12v you need both. If bottom switching you need just an NPN. For example if you have a common anode just connect it to 12 V with the cathode connected to the resistor and then to the collector, emitter to ground and base through a resistor to Arduino output.
i tested by directly connecting the segment pins to positive and a digit pin to negative.
It's a 12v sevseg common cathode.
Don't do that you will damage your digital pin as it puts 12 V on it when it is off which is way over the Vcc + 0.5V limit.That is probably the worst way round. You need an NPN and PNP to drive each segment.
how should i connect things with a common anode 12v sevseg?