You'll want a regulated 9 volt output. Just putting a resistor in series will drop some of the voltage, but how much voltage gets dropped will depend on the load you are connecting. If you are looking for a DIY approach, simple 3 terminal regulators like the LM7809 type are very easy to use. The three leads of this type of regulator are unregulated input, ground, and regulated output. Its also a good idea to to put a diode in series with the unregulated input, to protect the regulator and your 9V device in case you mis wire the thing. Some of these regulators require certain amounts of capacitance on the output and/or input. The datasheet for the actual part you use will ususally contain suggestions for the capacitance if it is required. Otherwise, there are off the shelf units to produce several different regulated output voltages from a car battery. They usually have a cirgarette lighter type plug and an assortment of different types of output power jacks.