No one can teach you the art of good writing. You must first have a desire to write well and then you must seize every opportunity to improve your skills. Only diligent effort can unlock the power of written expression.
While pursuing this goal you might doubt yourself—questioning your abilities. Put these doubts aside. Continue pursuing your goal with determination and you will gain a command of language that will clarify your thoughts and strengthen your ideas. You will make clear what other writers only manage to muddle.
Before attaining this goal, you will need to identify deficiencies in your writing and work to correct them. This takes time and persistence—but it can pay enormous dividends. Vast resources are available (in print and online) to help you along the way.
In particular, Elements of Style and Simple & Direct can remedy a host of writing maladies. A free Online Technical Writing Course by T.M. Georges will not only provide valuable instruction, but it will also lend purpose to your assignments as you focus on specific writing tasks. Finally, materials produced by the Advanced Writing Committee can help you expand and refine your vocabulary. Taking full advantage of all of these resources will likely increase the quality of your writing dramatically.
Yet writing will not be the only activity in which you will be engaged. You will also play a central role as "editor" and quickly discover that you know much more about editing and proofreading than you thought. There is power in acting as though you already possess a desired ability. This will become evident as you assume the position of editor and are asked to improve another's writing.
Ultimately, good writing conveys your ideas effectively, whereas poor writing obscures your ideas and confuses your readers. Your goal should be to communicate in such a way that you cannot be misunderstood. Writing well also enables you to examine, organize, and expand your thinking. Most importantly, the writing process is a conduit for "learning by faith" as you prayerfully learn how to summon ideas that lead you to greater insights and discoveries.