Pickpocket someone ! Or somethings... MORE legal: Wash some cars; run a lemonade stand; cut grass; rake yards; shovel; get a job; rob a bank... (oh yeah)
Like many others said, I would suggest getting a job or doing chores, obviously don't steal it, or sell drugs. Another way is if you have a bunch of toys or something that you don't use, try selling them; you could probably get $50 easily that way
If your neighborhood has a lot of elderly people, advertise yourself and do odd jobs for them, elderly people are always looking for young people to help them