'Elements of Style' (Strunk and White) is brilliant for threads/arguments and clarifying your thoughts, but don't forget those elements of creative style which make a piece of fiction so pleasing to read:

*keep it realistic, even if it's entirely fantasy. We respond to realistic reactions in fantastical situations, so use your imagination and your knowledge,

*keep it simple - don't elaborate action at the expense of interesting detail. Don't over tell story - that can be boring. Try to show not tell. Try to intimate what a character is like instead of giving a police profile. Try not to start with, (or revert to), exposition unless you can do it deliberately and stylishly.

*don't forget to describe how things look and how people feel/react - may seem very basic, but being short changed here matters to the reader.

*when detail is required, don't disappoint - (unless you judge that brevity suits your purposes better).

*play with words, phrases, constructs and always edit your work with an ear to what sounds right and expresses your idea best. (You should find yourself cutting down and tightening up/scrapping some bits and elaborating others.) The more you practice, the better you'll get.

Have you done your homework??