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. 
Drive for Uber or Lyft: If you're in a locale where you can find Uber or Lyft (or even one of the many competitors around the world such as China's Didi), you could easily make a respectable income. The hours are flexible and you can work as you see fit, making it perfect even if you currently have full-time employment but are looking to make some money on the side.
If you're crafty, you should consider selling your stuff on Etsy. Etsy is designed for handmade goods – including stationary, greeting cards, household crafts, and more. The great thing about creating these types of products is that you can do it in your spare time, on nights and weekends. And creating many of these products doesn't take up much space – so it can be dorm room friendly. Don't think there's much money in selling on Etsy? Well, this teenager paid for her college by selling crafts online.
Beyond selling notes, most college students write papers…lots and lots of papers. Don't let these assignments go to waste. There are services that will buy your old college papers. For example, you could look at GradeSaver. This services allow you to sell your old essays and get anywhere from $15 to $100 for them. If you're writing multiple papers per semester, this could be a great little side income.
How much will you make? I think my best month with Google AdSense was almost $5,000 over the last ten years. That amazing month blew my mind since it was actually near the beginning of my blogging journey. When you go from making zero to $5,000 in a month, that will rock your world. For me, it also got me even more excited because I knew there were other ways to monetize.