Resistance calculation with specific load
I want to start a device which required 0.5 amps and 5 volts.


If all you have is a 12V battery then you need to add a series resistor that drops 7V (12V then becomes 5V) whilst allowing 0.5 amps to the load: -

\$\dfrac{Voltage}{Current} = \dfrac{7}{0.5}\$ = 14 ohms (this is basic ohms law).

If your device always takes 0.5 amps then this will work OK but, if you require 5V to be delivered to your device when the current taken by said device is different then you need a voltage regulator.

Incidentally, whether it's a dropper resistor or a linear regulator the power dissipated in that component will be 7 volts X 0.5 amps = 3.5 watts.