Get a copy of The Elements of Style. It's like K&R for writing. Then practice. Join a local writing group. Write essays. Keep a journal. Write letters to the editor or guest columns for the local paper. Blog. Etc. Like anything else, the way to learn something is to do it.
Edit: also read a lot, and try to identify writing that resonates with you, and then work out why that is.
The Elements of Style is to K&R what The Voynich Manuscript is to On the Origin of Species. Elements of Style is good to read to see one person's ideas on style from a century ago, but it's not a guide to writing.
If you have a need for guidelines but don't want to develop your own, many major publications publish style books. For example: The Associated Press Stylebook, The Chicago Manual of Style. APA and MLA work too.
Edit: also read a lot, and try to identify writing that resonates with you, and then work out why that is.