What formulas do I need to use to design a voltage regulator circuit for a zigbee radio?
One Option...

The LM317T is a great place to start as it is basically indestructible and stable in the face of almost anything a student or hobbyist might do to it.

The part I recommended is the TO-220 packaged version. It's big and easy to clip or solder to.

Design Data

If you follow my link and click on "Documents" in the left-hand menu-bar you'll have instant access to all the datasheets and related documentation. On page 9 of the datasheet there is a whole section discussing tuning (set resistors and input/output capacitors).