I think many have already suggested several ways to earn money online. I want to add my opinion.
Start your blog on WordPress, write engaging articles according to your niche, or if you don't want to spend lots of time in writing, then create a page titled "Write for Us" and add content to it. After a few days, you'll see that you will get a couple of mail regarding guest posting. Select the best one and post it on your blog. And when traffic will increase on your website, then go for the Google Adsense program. If Google doesn't approve your website, then there are other websites that offer the same offer as Google does.