There are many ways to earn an income with online courses.
You can set them up on your own website, as a paid email course, or on a special website that promotes courses like Udemy. You can earn money from a course directly, or you can make money on the back end (or both).
You can earn money directly with a paid course. There are course websites like Udemy, Fedora, and many others. Look for “sell your course online” and you’ll find a lot of them to help you. You can also set up your course on your own website using a membership website feature like AWeber, InstaMember, or even just a PayPal button with a password only website.
You can still earn money with a free course. By giving your audience a short free course via email, or in a membership on your website, you can use the course to build your email list. You can then promote other things to them via the course and the email list. For example, on each email that you send about the course, you can include a link to your paid courses.
Remember that for any page on your site (such as the page where you send people to collect the next free module of your course), you can put recommended products that match things that your audience will need if they are to become successful at whatever you are promoting through your course.
You can earn money through all three methods if you want to. It doesn’t matter if a paid course is delivered or not; you can still add complementary products to the delivery page, or via an email newsletter that the students sign up to receive.
For example, if you are selling a course about how to write and sell a book on Kindle, you can promote services related to the venture, such as contractors who will do the work for them. You can also promote software that helps them, or peripheral products that they may be interested in.
A series of courses are an excellent way to earn more money through online courses. This works very well if the first course is free, then to learn more they have to pay for the upgraded future course.
You can always make money with courses. You just have to think about what your audience needs outside of the course, and you can promote those things to them as well. You can make money from the course directly, and then make money indirectly by promoting complementary products, services and more courses.