To be quite frank at this point in your life, you're not going to be able to make £500 in one month.
If you just want to make and save some money, see if any of your neighbours need odd jobs doing, could also try any/all of the shops near where you live, explain you can only work at weekends and see what happens.
Use your common sense - if it was possible to make £500 in a month when you're 15.. everyone would be doing it..
What do you need £500 for anyway?
Work for a pharmaceutical company and test their drugs, preferably neurological ones which claim to enhance the brains logical thinking - that if there was a simple way to make £500 in a month, everyone would be doing it.
What??? Who warned you your mum? Or this site??? Write how you want love, screw em - say it's your religion!
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I'll pay you £500 if you get an A in English.