If you're serious about making money online, start a blog. Blogging is one of the easiest and most sustainable income sources. As long as the blog is setup the right way, in the right niche, with the right content targeted at the right audience, and the offer is complementary to the content, you could make a tremendous amount of passive income from a blog.
Webinars are quite possibly one of the most potent ways you can make an exorbitant amount of money online. Russell Brunson often says that if you do a webinar every single week for a year, you'll be a millionaire at the end of it. You'll need an audience to train and you'll need to know what you're talking about. Of course, this usually requires having a website and some semblance of an online presence.
It doesn’t pay much, but if you’re a healthy person and want to make a bit of extra money online, the AchieveMint app will reward you for doing things like walking, tracking your food, or taking health surveys. AchieveMint works by connecting to fitness apps you might already be using like Fitbit, RunKeeper, Healthkit, and MyFitnessPal and then giving you points for certain actions. For every 10,000 points, you earn $10 with no limit on your earnings.
Upwork: This website offers a great marketplace for selling just about any professional service. You don't need a merchant account, website of your own or anything else for that matter. All you need to do is be able to provide a high-quality service at a reasonable price. But be informed, you will have to compete with many others that are constantly bidding on open jobs.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.