Pedal keeps coming loose on mountain bike?

I walk alone

New member
Bought a mountain bike about a month ago and all I've used it for is going to and from school. It's taken no knocks, falls or anything but the pedal keeps coming lose (the bit that connects the pedal to the main axel come loose) and it makes a creeking noise once every pedal rotation. This has already been fixed once for free but it's come loose again. What can I do?
 
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