Peace upon you,
From an early age I have been an avid reader. As I aged I had more and more people compliment me on my ability to write fluently, coherently, and correctly - despite that I failed every English class I took.
I suppose that, for myself, the foundation for a powerful author is, in fact, reading. However, not any text will do! I have always attempted to surround myself with outstanding examples of literature. On my fiction shelf, just as an example, I see the Lord of the Rings series (and anyone who argues that J.R.R. Tolkien's masterpiece flies in the face of fine writing is someone I would choose to, respectfully, ignore), a compilation of Edgar Allan Poe's works, Bram Stoker's Dracula, His Dark Materials (by Phillip Pullman), Great Expectations, and a few others such as C.S. Lewis' 'The Chronicles of Narnia' series.
Of course, a thorough knowledge of your subject matter is necessary. I am an ordained Reverend of the Christian Church. However, I do like to study outside of my belief system. On my desk, next to me, I have a copy of the King James version of the Bible, an Artscroll Tanakh, an Artscroll Siddur, a Koren siddur, and a Qur'an.
I have composed a few minor apologetics, essays, and other works. I would recommend starting small. Step one, of course, is simply to begin writing. Anything will work! Start just by typing things about yourself. My name is Shaeykh and I am 23 years old, for example. Get yourself started and see what flows from you. I'd say that you are not at a point where you can have something on your mind and expect to channel that fluently into coherent thoughts on a page. You need to build to that!
After some nights of practice you should see serious improvement in the fluidity in which you compose your thoughts with pen and paper. Now you find yourself turning your efforts to a minor apologetic or essay. Think of something you wish to speak on and simply compose a simple, three or four paragraph work. In the introduction you present the purpose of your writing and what you intend to prove/deny/etc. In the next paragraph present your argument, then in the last paragraph tie it all together and reiterate what you spoke on.
When you're done start reading it forwards. Look for mistakes, inconsistencies, spelling errors, etc. Then read it backwards. Then write it backwards. All of this is to help you polish it up.
When it is ready, present it to a close friend and ask for their most brutual advice and criticisms. Be open-minded! If you feel insulted and close yourself off your career as an author, of anything, will be very short-lived indeed!
I will keep you in my prayers and I wish you the very best,
~ Shaeykh