My freshman year writing class in college helped a great deal. The only book we were asked to buy was a short book called "The Elements of Style" by Strunk and White. There are summaries available online, but try to get a copy so you can see the example sentences, and how the authors re-wrote them. That alone will give your sentences a boost.

It helps to know what you want to write about. In my class, we wrote short articles on the effect of the internet on economy and society (very long ago, pre-social media). The 4-month long period of writing articles, getting feedback, then writing more articles, getting more precise and actionable feedback ultimately got me to a better place.

My instructor nitpicked everything, as she should have. One of the red-ink sentences I remember from my very first graded work was:

"Why does your article have this tone? What were the unspoken assumptions you made when you wrote this? Put them at the top of the article so that the reader is not left to speculate". I always remember that because writing on social media works the exact opposite way!