new MTB creacking on hard pedaling
If only one side is creaking, it's probably the pedal or crank arm not screwed tight. So first tight them up (for the pedal you probably need a special wrench, for crank arm it's a 8mm hex or 14mm wrench). You should tight it as fast as you can, as it can brake the crank arm.  

When both sides are creaking, it could be that both sides are loose, but it also can be a problem in wheels i.e. spokes are loose, or hub is broken, or it can be the BB.  

Those are the much common problems, but as @mattM linked at sheldon brown, it can be other things as well.