Your style will develop and improve the more you write.  But in my experience the stylistic weaknesses of student writers tend to fall into several general areas, all of which can be strengthened if attention is paid to them.

Verbs.  The quickest way to improve your writing is by paying attention to verbs.  English is a language rich in verbs, so the use of vague verbs or long verb phrases is unnecessary.  Simply converting "takes a look at" to "examines," "talks about in detail" to "analyzes," "sneaks out of" to "escapes," etc. will make a difference in your prose.  Weak action verbs (such as do, get, have, go, make, say, etc.) can often be replaced by stronger ones ("has," for example, can be replaced by words like "displays," "exemplifies," "demonstrates," etc.).   Weak action verbs paired with an adverb can usually be treated similarly ("talk heatedly" becomes "argue," "go slowly" becomes "shuffle" or "dawdle," "beat badly" becomes "annihilate," etc. ), as can those paired with a noun ("make an argument" becomes "argue," "give an explanation" becomes "explain," etc.).  And as much as possible, make sure your verbs are in the active voice: "He threw the ball" (active voice) rather than "The ball was thrown by him" (passive voice).  As you can see, passive voice is wordier and less direct.

Sentence variety.  Make sure there's some variety in the length and types of your sentences.  Work at occasionally opening a sentence with something other than the subject.  Student writers often tend to write strings of short, simple sentences.  This makes the paper choppy and makes the ideas sound simpleminded.  If choppy sentences are a problem, combine some of them into longer and more complex sentences.

Wordiness.  Make your prose clear and direct--remove all unnecessary words.  Say "because" rather than "due to the fact that," "is" rather than "is seen to be," "would be" instead of "would prove to be," etc.  Watch out for redundancies.  One of the most common in student writing is "the reason is because"--"because" means "the reason."  My favorite came from a student who wrote, "the teacher stood up in front of the class and lectured."  "Lectured," however, means "stood up in front of the class."  When you're editing your paper, ask yourself if each sentence could be simplified.

Grammar

College professors maintain the annoying belief that native speakers of the English language should have, after twelve plus years of education, a basic mastery of English grammar.  If you do not own a grammar handbook you should get one, and you should use it.  Grammar matters not because it's an arcane and impossibly-complex system of rules designed for the torment of students, but because it facilitates the communication of what you have to say.  Grammar and content, in fact, are almost always related: when the prose isn't clear, neither are the ideas.

When grammatical errors are marked on your papers, it is your responsibility to understand the problems and correct them.  Consult a handbook, and if that doesn't help, please ask--either me or a tutor in the Writing Center or someone else.

I use the standard correction symbols found in any grammar handbook.  The ones I use most frequently are: