I'm not the greatest mechanic but generally speaking, things point to a slightly loose connector (bolt) somewhere in the crank, and the solution is to take the crank apart and clean it and grease all the nuts and bolts and tighten them close to the specified torque. It's a cheap bike and might not have been assembled with the greatest to detail.

Possibility #2: I'm not joking, the problem might be in the saddle or saddle seatpost connection. If you stand up and pedal and the creak goes away, then the problem might be in your saddle area. Same procedure - take it apart and clean it and grease all the bolts and tighten them to a reasonable tightness.