As Mike says, you need to understand the difference between power supply level and signal level.
You need the regulator for power supply. There are then numerous options for signals. If you have a 5v signal going into a 3.3v device, a potential divider will probably work. Alternatively, a transistor or a mosfet. In the opposite direction (3.3v signal into a 5v device), in theory, the 3.3v is enough to give a logic high - it's 66% of 5v after all.
If all else fails, have a Google for 'mosfet level shifter' or 'signal level shifter' - or buy the board of a similar name from Sparkfun.