Creaking cranks are usually the result of a loose or worn bottom bracket. Tightening the crank arms themselves will not fix the problem. The arms need to be removed and the bottom bracket cups checked for tightness. Removing and greasing before reinstalling and properly torquing would be even better. The bearings can be assessed at this time as well. If it's a cup and cone bottom bracket, it can be disassembled and regreased and if needed have fresh bearings installed. If it's a cartridge style bottom bracket, you just replace the entire thing when it's worn out.