The trend goes towards content marketing on the platform of your choice. The platform of your choice: wherever your customers are.
Everyone has something to market: themselves. (Follow Justin Welsh (linkedin, twitter) )
What is your background? What is your experience? What are you passionate about? What value van you provide for other people.
I don't think there is something like a "hello world" for online marketing tbh. Content marketing is only one aspect on online marketing but the easiest to get started with. It costs only time; it also takes a lot of time. You need to have at least 3-6 months till you see some results and need to be posting regularly.
For a more advanced and challenging option, set up a Shopify site and try to sell something there. Buy Google Ads to get traffic to it. Blog writing will likely take months to pay off unless you have some kind of audience you can get it in front of.
For a really advanced option (that almost nobody will take you up on), find a local business and make them a website that’s optimized for SEO. Get them listed on Google Maps, onto directory/citation sites, and start tracking conversions through Google Analytics. I was fortunate to be able to do this for my partner’s e-commerce business and the lessons I learned were really valuable starting my digital marketing career.