Need help with stepping down DC volts
You have a 19 V power supply, but what you want is a 12 V power supply.  Instead of converting the 19 V to 12 V, just get a 12 V power supply in the first place.  That's a common power supply voltage, so there will be lots of choices.  Then you still have the 19 V supply to do other things with.

Using a resistor to drop the voltage can work if you know the current well, but it will waste a lot of power.  The extra power usage may not be a concern, but getting rid of the heat may not be so simple.

You want the resistor to drop 19V - 12V = 7V at 4.3 A.  From Ohm's law, that would required a (7 V)/(4.3 A) = 1.6 &Omega; resistor.  The power dissipated will be (7 V)(4.3 A) = 30 W.  Not only does the resistor have to be rated for that, but you have to allow 30 W of heat to be moved away from the resistor.

Look up how big such resistors are, what they cost, and how you have to mount them to allow the 30 W to be removed in steady state.  The plain old 12 V power supply will look pretty good about now.