Drive for Uber or Lyft: If you're in a locale where you can find Uber or Lyft (or even one of the many competitors around the world such as China's Didi), you could easily make a respectable income. The hours are flexible and you can work as you see fit, making it perfect even if you currently have full-time employment but are looking to make some money on the side.
If you're not self-employed and work for a company, find out if they have a retirement plan. If you're lucky, employers will sometimes match contributions you make into a retirement fund. Retirement plans also often have the benefit of being tax-deferred. The longer you get to keep your money (and make interest on it) the better. It's never too early to start planning for retirement.
If you’ve got some free time and don’t live in the middle of nowhere, becoming a Lyft driver can be a very lucrative side hustle that allows you make money fast. And right now, they’ve got a promotion going on where any new driver will instantly get a $300 bonus after completing their 100th ride. If you start now and hustle hard on the weekends, you can probably unlock that bonus within a few weeks of driving (and that’s in addition to your normal earnings).