Udemy is one of the world's largest platforms for selling online courses for just about any topic or subject. As long as you have the proper skills, you can likely teach those skills on a platform like this. However, don't expect it to be easy. Fierce competition means that you need to deliver loads of value and expertly market your course online if you're going to make any respectable amount of money.
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.