Well first take the cranks 100% apart including taking the spindle out of the BB put grease all over the splines of the cranks and the entire spindle. Put the spindle back into the BB (bottom bracket) and put the cranks back on and tighten them down. Make sure the dust covers are facing the right way and tighten the sprocket bolt down nice and tight. If it still squeaks, lube the chain. If it is still squeaking after all that just ride it anyways. Or if it is annoying enough it may even be the Bottom Bracket bearings, if so remove the BB bearings. Take the seals off the bearing and be careful not to damage either seal. Place the Bearings in a bowl of rubbing alcohol and clean them and then take grease and grease the crap out of them and put the seals back on and put the bearings back into the frame. If it still squeaks it may be the pedals or something. If it is just buy new whatever it is squeaking.
References: OneGuyIlluminatiEye, robinson79, Brian Griffin, The Horror Contact, StoreBoughtChild, C_Johnsonbmx, dkTechEthan, etc.