Voltage Regulator 317 - Selecting Resistors ( R1, R2 )
You want to select resistor values such that the current through them is much greater (at least 10X) the quiescent current of the regulator itself. Resistor R2 passes the quiescent current of the regulator as well as the current coming from R1.

The quiescent current may also be called the "adjustment pin current". A typical value for R1 is \$240\,\Omega\$ which gives a current of about 5mA through the resistor divider. For the TI LM317 the adjustment current has a maximum value of 0.1mA, so the divider current is about 50X bigger than the adjustment current.

The formula provided to set \$V_{out}\$ is based on the notion of a resistive voltage divider, where you have 2 resistors and by selecting the ratios of the resistors you can set the voltage at the point where they are connected together. Connecting the adjustment pin of the LM317 to the resistor divider injects an additional current into the bottom resistor so the divider formula isn't quite right. However, if you choose resistors so that the current from the adjustment pin is small compared to the current that would normally flow through the divider then the error can be minimized.