if you're getting one creak each time you put a pedal forward and put significant weight/power on it, then odds are its the crank to BB interface.

the usual solution is removing the crank, cleaning the BB-crank interface, and then reinstalling the crank. assuming this is a square taper crank, the center bolt has to be torqued on pretty hard, like 60 ft-lbs or something. (look it up at the park tools site)

if its the BB itself, well, then it depends on what kind of BB it is. if its a cartridge BB, removing it, cleaning the threads on the shell and the BB, thoroughly re-greasing, and reinstalling, being sure to torque the lock ring on hard enough is usually sufficient.