Step down 20V DC to 9V DC
A voltage divider made up of resistors isn't the best way to drop your voltage because if the current drawn by the load changes, the voltage across it will also change. (Do a basic search on resistor voltage dividers for more.)

There are two primary ways to regulate or drop voltage: linear and switching.

An LM317 for example, is an adjustable linear regulator which can easily regulate 20V down to 9V provided you aren't exceeding its current rating of 1.5A.

From the datasheet, here is a schematic of typical usage:



However, linear regulators such as this produce heat, and are not the most efficient way to step voltage down.

Switching regulators (or a buck converter) essentially switch on and off at a high frequency and provide regulated voltage with greater efficiency. There are all-in-one packages such as ST's MC34063ECN. I picked this example because it has similar current handling as the LM317, is inexpensive, and is also a through-hole component in case that's what you're looking for.

Again, from the datasheet, here's a schematic of typical step-down usage:



As you can see, it is a little more complicated, but worthwhile if you need greater efficiency.

Finally, don't forget that switching power supplies might be an option. Unless you need to derive 9V from a DC source, you might simply consider a 9V regulated switching power supply that gets power from the mains.



Edit:

I should also mention that an LM7809 is a fixed 9V linear regulator which would require fewer external components. However its current limit is 1A. An AVR development board likely won't require that amount of current unless you're also powering some external components.