fast is relative to the needs in your area...
for the most part it requires going outside,
and either going door to door (resealing driveways is popular for some)
or hanging with the other bums for day workers
one the slowest turnarounds but potentially most profitable with little investment is upcycling and refurbishing. which for the most part does better when you can sell locally vs dealing with online competition especially in a world market. I do a bit of mending, revamping and refurbishing from electronics to apparel & furniture. the latter is the most profitable for me but the turnaround is slow, largely seasonal in these parts... and requires work space & storage. but there's a lot of lazy or self doubting people that would rather buy than DIY. this goes the same for many services as well.
online businesses - there are options, there is also a lot of competition, but it has it's peaks and valleys.. it doesn't really have to cost you anything to start but you're better off with some investment, particularly if you don't have a fairly large following on social media. . . and you're lacking creative skills, however there are several sites you can commission work for. There's also article sites, new aggregator or whatever, really. there's uploading and ads but that usually ends up in gray legal territory. there's offering services, many options. if you're adult, well, there's things that require being an adult - not all dirty, but remember there is a fetish for everything. nearly everything requires seo, advertising and social networking, though. it's not enough just to put it out there.
reselling is a popular as is re-labeling. but quite a bit more of a learning curve, even if you don't have to officially setup a business with it starting out.
there's many options but it largely depends on you, your resources, skills, time and effort.
perhaps the most profitable and quickest turnaround is getitng into allied health fields & nursing. the demand outweights most everything else. so, there is a lot more funding available and for many no degree is required, however, they still require education and licensing. outside of tech, there's many other certificate and associate programs you might look into exploring. it all really depends on your interests and capabilities.