Inductor calculation for DC to DC converter
A really great walk-through on DC-DC converter (Buck topology specifically) design calculations can be found at this Microchip Technology's web seminar slideshow. The seminar covers these important topics: 


Calculate the required inductor
Calculate the output capacitor requirements
Select the input capacitor
Select the diode
Choose the MOSFET
Calculate the converter Efficiency


You need to know things like the input voltage, intended output voltage, acceptable input and output ripple voltage for your system/converter IC, and current requirements, as well as the switching frequency of your DC-DC converter IC.