Quick and dirty,
Google LM317 & LM337, download and READ the data sheets. Linear Voltage regulators are not real efficient, but they ar very easy to use (if you follow the rules).
Level shifters: easiest way is to look up some on Sparkfun's web site and look at the schematics. Or Google I2C Bidirectional Level Shifters. Look up Philips Electronics I2C-Buss Specification (Page 43 in particular, try 1.2Kohm for R1-R4).