Whether it’s an important consumer application, a specialist app to solve a particular niche problem, or even a time-wasting game you can play on your phone, you can create a massively successful business if you build software that helps people. (Look at the rise of Slack—the team communication software that went from side project to billion-dollar company in just 2 years.)
For example, if you buy a $100 suit… perhaps you could rent it out for $25 for the night/weekend and someone locally is interested in just a cheap rental (because they don’t need to own a suit for the one or two times per year they wear one). After four weekends of renting the suit, it’s paid for itself. Now, whenever it’s rented out—you’re profiting for the remaining life of the suit.
Find a profitable niche. Starting with your interests, write down as many niche ideas as you can. Think about topics people might search online. Ideas include passions (like surfing or body building), fears (like spiders or speaking in front of crowds) and problems (like getting out of debt). Do keyword research to see it others are interested in the topic. Find out if a domain name is available that matches the keyword 100 percent. 
If you’ve got a way with words and expertise in a niche, there are plenty of sites that will pay for articles and content you write. Think of the sites you read regularly. What can you contribute to them that would be interesting? Research your niche and then look for ways to pitch articles. Many sites will simply have a submission or contact link in the footer. To get started, check out my full guide to becoming a freelance writer on the side and then submit your articles to places like Instash, Listverse, A List Apart, International Living, FundsforWriters, and Textbroker.
Paid traffic, such as PPC or banner advertising, is usually more controllable and scalable. If what you're doing is working, you can spend more to grow your traffic. It also has the advantage of being able to start very quickly, if not immediately. The main disadvantage of paid traffic is that it can be quite expensive and requires a lot of supervision to avoid losing money. Paid advertising platforms such as Google AdWords can also have strict requirements regarding advertisers and ads, so be sure to pay close attention to all rules and warnings.
You'll also need ecommerce software, fulfillment software, worry about warehousing, customer service and refunds. But that's not all. You'll also need traffic. Think search engine optimization, Facebook ads, and other social media campaigns. It is hard work, especially on your own. You could opt for Amazon's platform, which might be the easier route. But, then again, at the end of the day, this is a serious business, which could produce significant profits. So you're either all in or you're not.
Create a killer course experience: With your course validated and in the works, you need to figure out how people will take it. Most course creators choose to host their courses (after going down the path of learning how to make a website) on their own blogs. This way, they get all the value of bringing customers back to their site on a regular basis. I host my own courses from a subdomain on my own site so I can easily add more. The course experience is incredibly important as well. And after trying most of the solutions, I highly recommend Teachable—an online platform designed specifically for courses.
When was the last time you went to a new restaurant without looking it up online beforehand? Or bought a product that didn’t have at least a few 5-star reviews? It seems like more and more our world is run on reviews. And you can make money online by writing them. Get started by creating accounts on sites like Vindale research, Software Judge, FameBit, CrowdTap, Influence Central, and Modern Mom. However, before you run off and start writing, be sure to check the small print on each of these sites. Writing reviews isn’t a huge source of guaranteed income and you want to make sure that it’s worth your time before you get going.