Okay.....you'll need a handful of tools for this one; Remove the crank bolts with a 15mm socket, then remove the crank arms with a crank puller. Remove the BB lockring from the non-drive side of the BB with a BB spanner, then remove the BB through the drive side of the BB shell. Clean all of the threads in the shell very well as well as the BB itself. Grease the threads in the BB shell and reassemble.
My experience is when the interface between the BB shell and BB gets dry, a creak develops. Now, all of these tools will cost you more than taking your bike to a shop, but I'm a believer in learning to maintain one's own bike. Good luck.