multisized 7-segment displays in a circuit
I agree with Majenko there are better alternatives but in answer to the question you need an interface circuit that will change a 0 -5V signal into a 0 -12V signal.  This can be easily done with a few discrete components. 

This is the sort of circuit (but by no means the only one) you could use. A google search will throw up many others including the use of MOSFETS, open collector gates etc.



Q1 and Q2 are just about any general purpose small signal type  transistors - Q1 is an NPN type and Q2 is a PNP type. The resistor values are not critical so anything in the 10s of K would be suitable. 

R1 limits the current to the base of Q1. An input voltage above 0.6V turns Q1 ON. This pulls current through the emitter-base of Q2 through R3 (which limits the base current of Q2). Q2 is turned ON and current flows through Q2 and R4 raising the voltage across R4 to nearly 12V. 

With Q1 input around 0V, Q1 is turned OFF and no current can flow through R!,R2. The resistor R2 ensures that Q2 is fully turned OFF. With no current through Q2 there is no voltage across R4 and so the output will be 0V.