Newcomers to writing nonfiction research—often new PhD students—may find the research process and the formal structure of research documents daunting. Here are a few resources both on research writing and on the writing craft itself that I found useful when I was starting to hone my skill. And, yes, writing is a skill; you can improve your writing through practice.
Above all, seek out and be open to feedback. Anyone who is willing to offer honest feedback on your writing to help you improve it is valuable.
The Craft of Research by Wayne Booth, et al
A Manual for Writers of Research Papers, Theses, and Dissertations by Kate Turabian, et al
On Writing Well by William Zinsser
The Professor is In by Karen Kelsky
Avoid Strunk and White. It’s an interesting historical read, but much of the advice is either incorrect or outdated, by modern standards.