good job, why don't you try like several of us do/did by becoming a computer service man (don't know the right call for the job

), just spread it out in your school and your neighborhood, have a namecard/leaflet ready.
maintain good face and relationship, you'll have additional income by servicing computer...
Well if you want a job that you can do from home, search for envelope packing, this just means that you seal envelopes and earn money: eg: you can earn $1565 is a week ! just 200 envelopes per day....something like that ... i read it in a BLOG somewhere.. LOL, should be there in most countries LOL

just a suggestion

Yeah, servicing computers is a simple and effective idea. Since many already have computers but don't even know how to keep it clean. So servicing and cleaning services would be welcome. And if they don't know it is important show them (educate them) on what would happen if dust accumulate within the PC.
There's a photo here somewhere about how dusty a computer can be.
Haha you remind me of me. I bought this within days of getting my first real job (about 2 weeks ago):
NB: I cheated, Dad got me a job with one of his clients.
lol , i heard you can earn $$ by reading ads..
Most of those "reading ads" money-makers are fake. I tried a couple back then.
That a lot to ask for when your 15 year of age, i mean you need some good skill's i mean of you good with web design go their, make web template's sell them, make logo's do general Design's i was selling logo's for $1:50 with good design's and made, and got at least 2-5 buyer's a day or week depending when i took the request, if you got a good skill, that your best bet! With what i was doing i still wasnt making close to 100 bucks a month.
Well if you listen to the "internet marketing gurus" they'd tell you that a 10 year old can make USD10k in a month. But what they don't tell you is how they did it
Bought some of these internet marketing books and they talk about drop shipping which is do-able for a 15 year old. But you have to get people to pay to your parent's banking account unless you have your own paypal account or clickbank account. So if you read up about it, 500 a month is do-able. But it takes time - reading and understanding.
$500,- are you on drugs...
Indd like zach said a real job would be better specially if your income now is 0 it wont get 500 fast.
Gerard go and ask your parents for the money! this will solve the whole issue.
or of course you can always hit a bank!

If you have any skills that you could offer people such as writing articles, designing banners, etc, you could make some money pretty quickly. This isn't business building, necessarily but then you didn't say that you want to build a business right now. One place you could go to offer your skills would be Digital Point, but there's also plenty of freelance sites where you could try like getafreelancer, etc.
Update on this thread.
I just got a job at
Swiss Chalet as kitchen help. I get about $80 working 3 to 4 times a week. It's small, but pretty decent for a full-time high school student.
Congratulations on the job. It might take a little longer to get that guitar than a month at that rate but you will get there. If you have the extra time, you still could try a few things that have been suggested here. You seem quite capable of stringing together a sentence so writing a few articles to sell somewhere would be relatively easy way to get to your guitar faster.