12v to 5v 1a
Hi !
The simplest way to do this is using a linear voltage regulator as 7805 (from National/Motorola/Fairchild etc) device. It will regulate the output voltage to 5V @1A. This device has only 3 terminals (Vin, GND and Vout) and needs only a bypass capacitor in output (100nF).
Another option is the LM317 (adjustable regulator), also for 1A, but you need more 2 resistors and the capacitor.
These linear regulators waste power, because they convert the difference betwenn output voltage and input voltage (multiplied by the current) into heat, so maybe necessary to use a heatsink attached to the regulator. Efficiency in this case will be 41%.
To get a more efficient regulator, you can use a DC-DC converter using for example the LM2576-5.0 (a siwitching step down regulator) that converts the 12V input voltage to 5V using a inductor, a fast diode and a filter capacitor. It has a much better efficiency - around 75 to 80%.
There other ways and devices, but these are common and simple.