Depending on what part of the world you're from, the law might differ... but generally 15 year olds can enter into part-time employment, so long as you are also in education or training until you become of "school leaving age".
That's the way it works in the UK anyway.... Here's some info on working when you're under 16 in the UK: https://www.gov.uk/child-employment
Even if you are legally allowed to work, actually finding work may prove difficult. Most employers seem to prefer employing people over 16 for a number of reasons...
You could overcome this through several ways though:
- Find somewhere that is desperate for help (the minimum wage doesn't seem to apply to under-16s as you are seen as an apprentice, so that may work in your favour... But be careful not to get ripped off!)
- Use your connections. This is probably your best bet. Ask friends and family to put in a good word for you. And if you do land an interview, make sure you come across as being very keen to get some experience and to get your foot on the ladder.
- My favorite suggestion would be to find something you have a specialty or skill in, and then apply directly to people who can help nurture that skill. Engineering this kind of situation is even more valuable than simply the money you earn, but it's not easy to come by. For example, if you are interested in making cool designs on the computer, and know your way around photoshop, you could get in touch with a design agency in your area - either through their website or on LinkedIn, and write a persuasive message saying that you're willing to work hard to get some experience in that specific field...
Final tip: It's a numbers game. Obviously it will get easier as you get older and gain more experience, but applying for jobs is literally about finding the right fit at the right time. If you keep applying far and wide, you're more likely to find something... So don't be disheartened by rejection letters, just keep going and you never know!
P.s. If you can give me an idea of the kind of things you're into/any skills or talents you think employers would appreciate, I might be able to recommend some more specific advice!