Sounds like something needs a quick tighten up.
Does the creak occur when the pedals are in the same position each time you turn them or not?
Most likely to be one of the following:
Seatpost.
Saddle
Pedals
Handlebar
Bottom Bracket.
Crank
Crankbolts
My suggestion at this point is to go around the bike, and nip up any of the bolts you can see. This should cover the seatpost, saddle, pedals and handlebar. You can also undo each one and add a little grease to them.
For the Seatpost and saddle the best way to check to see if it is these is to see if the noise persists when you stand up and pedal or not. If it does, it's not one of these.
For the pedals usually they have an allen bolt head on the axle where they screw in (behind the crankarm) if not then you'll need either a pedal spanner or a thin 15mm spanner if you don't have one. I'd take these off and add a bit of grease to the threads and refit them. They don't need to be bastard tight just comfortably tight will do.
Cranks and crankbolts are a bit more of a depends on what you have, so pictures are a good help here.