You definately don't want to use a voltage divider. Most people prefer a linear voltage regulator like the LM7805. Or use a switching regulator module. It would waste much less power. You will need to check the specs of your regulator for input/output voltage. Much depends on what you are using it for.
http://www.dimensionengineering.com/info/switching-regulators
Check this one out:
OR
If you're new to electronics and arduino, you'll be much happier and probably more successful with an arduino module with built-in voltage regulator.
Here's a link to a similar topic:
http://arduino.cc/forum/index.php/topic,23460.0.html