My advice would be to learn about SEO. There are plenty of blogs about it (Backlinko, Moz, Ahrefs, Search Engine Land, Search Engine Journal, SEMRush, Search Engine Watch, etc.) and there are affordable online courses on Udemy. There are also some marketing courses you can audit for free on Coursera (for example, this Introduction to Search Engine Optimization).
The type of writing gigs you're likely looking for right now will want you to have at least a basic understanding of SEO principles and how they relate to content creation. After you've got some SEO knowledge down, I'd write up some sample articles. If you have a topic you're interested in writing about (fitness, cooking, space travel, finance, whatever) I would draft a few articles for that niche. If you're willing to write about anything, pick some subjects that interest you and check out websites/blogs related to those topics. Think about an article that might fit on that website -- and then go write it. Once you've built a portfolio of SEO writing clips you can search for jobs online or set up a profile on freelancing websites like Upwork.