use simple resistor divider network with 1:2 ratio. u'll have to use very high wattage resistors, (depending on current required). or use 12 regulator of current rating as required by u.
If you need <1A then you can use LM7812 voltage regulator, if you need more current you will need to add an external power transistor (with a heatsink) .. see picture below ..
More details you can find at: **broken link removed**
It could be possible using a DC-DC converter(switching mode) it can supply high current with very high efficency and usually it doesn't need heatsink. Try to see Tracopower devices. I can't suggest you a specific IC but i know there are a lot of small DC DC integrated converter for low current. You have to search for 'step down' converter for example in Linear technology or Maxim sites.
the attached circuit diagram for 24v dc to 12v dc, can i still use it for my 19v dc output by only changing the LM7812 to LM7818 or LM7819, for example. what of the values of the resistor. my load current is 3A.
the source being battery in vehical, you should save the battery power, i mean, you can use resistor divider but it will draw some current. further you have to connect voltage regulator 78xx which will descipate additional power leading to faster drain of battery. instead you can use a non-isolated buck converter which has higher efficiency than linear volatge regulator.
check LM2576 Datasheet : **broken link removed** @ Page 10 you can see the Adjustable Version of LM2567 configuration (your input voltage is 24v , you can use regular version of LM2576-adj not LM2576HV-adj) change the R1 & R2 Values to give you 19v Output. it's a 3A Regulator. USE R1=1K , R2=8.6K+10K(POT) and adjust output voltage with the 10K POT.
for an easier config : download LT1083/10804/1085 Datasheet : http://cds.linear.com/docs/Datasheet/108345fg.pdf and @ Page 11 check the circuit. use : R1=120 & R2=1K+2K(POT) . adjust output voltage with the POT. Remember LT1083 is a 7.5A regulator , LT1084 is 5A Version and LT1085 is 3A Version. and if you are willing to use Linear Regulators Like these regulators for high Current you should have a Very good Heat-Sink. For Example : if you want to use these regulators for 3A , your regulator will Dissipate 15W. >> (Vin-Vout)*I=Pd >> Pd=(24-19)*3=15W , so you need a good heat-sink. you won't have this problem with the switching regulator (LM2676) at this rate (but it will need the heat-sink too , but for lower dissipation. for example around 3W~4W.)