There are many legitimate ways to earn extra cash online. Writing is definitely one of them, but it will not happen quickly. Either a blog, or other writing sites will take time to build up content and readers to get the ball rolling and keep it rolling.
You could also try day trading or currency trading. But again, there is a lot of knowledge that is needed to get started, and the risks can be very high. There are days where you gain over $100 and days when you lose over $100, and that is with a very small starting amount(around $1,000).
Affiliate marketing can work, but again is not a get rich quick scheme.
The thing to keep in mind with any idea online is that while it may be very convenient to work from home/online/anywhere, that also creates other barriers of entry, such as a flood in the market, or many people or websites doing the exact same thing.
The possibilities are endless, you just have to come to the realization that it will not happen overnight, and for the first few years you will work way more than 40 hours a week to later work less. Just remember that if it was easy to do, everyone would do it, and where there is 1 success story, there are thousands of failed ones.
Hope that helps!