Airlock: indeed, you do not need it for the fermenting process, it is added as a way to insulate your brew from the surrounding air, to make sure that no contamination happens, and to remove the CO2 once your fermentation is under way.
moulds, spores and bacteria could enter your beer. I suppose you did add the lid, so that only the bunghole ...
Is it bad to produce beer like this? ( i don't have money to buy airlock).
I wouldn't call it ideal, but it is fine. The only purpose of an airlock is to allow air to escape and not enter.
Should I open the bottle every 12 hours to let the gas come out?
Yes, I would open it to let it degass to prevent the bottle from exploding during the few days of ...