It is probably the BB, crank bolts, chainring bolts or pedals as suggested but always verify if the creak is still there when you pedal standing up. This eleminates the possibility of a problem with the seat post or rails. Another somewhat remote but possible source is the handlebar/stem connection. It is generally wise to go thru this process of elemination before starting to pull the drivtrain apart. On my last 4 'creak problems' 2 were bottom brackets and 2 were seat posts. Good luck!!