Study and learn correct grammar. A paper filled with grammatical errors will likely earn you a failing grade. Additionally, in college you will be asked to provide short-answer responses on in-class exams, and if your responses have grammatical errors, those errors will bring down your grade.
You can learn correct grammar by studying a grammar book. Complete the practice exercises and learn from your mistakes. Pay special attention to the sections that deal with these common errors:
1. Subject-verb disagreement
2. Comma splices and fused sentences
3. Sentence fragments
4. Pronoun-antecedent disagreement
5. Shifts in verb tense within sentences
6. Split infinitives
7. Faulty parallelism
8. Misplaced and dangling modifiers
Increase your vocabulary. Granted, it's possible to get ideas across without using "big" words. But college students are expected to have a more advanced vocabulary than high school students. If your vocabulary is limited, try to increase it by learning additional words, as well the meanings of unfamiliar words that you come across in reading or conversation. Learn by looking up definitions and pronunciations in a dictionary, then use these words in conversations.
Develop critical thinking skills. As John Chaffee, author of "Critical Thinking, Thoughtful Writing," says, "No collection of writing tips and strategies will enable you to write thoughtfully if you're not thinking critically."
To apply critical thinking skills to your writing, learn to organize content into logical patterns. For example, some information is best suited to a chronological arrangement, while other information lends itself to a cause-and-effect arrangement. Also learn to present evidence, details and examples to support your arguments and conclusions; demonstrate an understanding of complex material; show connections between events, especially in history classes; and avoid illogical or fallacious reasoning.
Practice communicating ideas clearly, precisely and competently. Every semester, students fail courses because they can't express their ideas coherently.
Communicate well by choosing the most effective words, rather than the first words that come to mind; eliminating unnecessary or redundant verbiage; using a variety of sentence structures; and using correct punctuation.
Use your English skills in conversations. If you make a habit of expressing yourself poorly in everyday dialogue, chances are you will also express yourself poorly in writing. Even casual conversation is a chance to practice communicating well.