you can use a 5 volt regulator from radioshack. they have three pins on them input (batt probably), common (0v ground), and output. They have current limitations though, commonly 1 Amp, but that is a lot in microelectronic terms. They work by shedding energy so it may be advisable to put them on a heatsink (the make small ones for to-220 package ic's or you can do as i did and get a piece of aluminum with thermal interface tape).
If you need a lot of power, get a DC-DC converter from www.mp3car.com
, as mentioned.
Don't just use a resistor...the voltage drop across a resistor changes with current.
I don't believe you can use a step down transformer...those only work with ac currents.