I have a squeaking problem on my bike?

Ok so whenever i am pedaling my bike, it squeaks. What happens is that as my left pedal is pushed from the 12 o clock position dowwn to the 6 o clock position, something around the pedal squeaks. My right pedal doesnt do this at all. My bike IS old, (firenze GL5000) so maybe thats it. But does anybody know why my left pedal is doing this and what i can do to fix it?
 
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