Whether you're learning English as a second language or are a native speaker who's never had the knack for writing, improving your English composition skills is very important. Knowing your shortcomings and practicing will allow you to write prose that is at first simply grammatically correct, and eventually sparkling with style. To do this you'll need to read more, ask friends for help, and continually write. Practice does indeed make perfect.
Broaden your vocabulary by reading as much as possible. Look up the definitions of words with which you are unfamiliar. The more words you can use successfully, the more often you'll be able to find the right one for the task at hand -- and the less likely your writing will be simplistic and repetitive.
Ask a friend with good English skills to critique and proofread your writing. Knowing what you're doing wrong is necessary before you can improve. Rewrite your paper or letter after listening to her advice, making the necessary changes.
Use your word processing software spelling and grammar-check tools to fix simple errors. Whenever you misspell a word, replace it with the correct suggestion and then look at it carefully, noting the differences from how you thought it was spelled. Most modern Web browsers also include spell-checking tools. Use them.
Think in English while writing if you're a native speaker of a different language. Having to translate in your head slows you down, and can easily lead to errors. Study English grammar if you are having difficulties. Look up the rules whenever you're having difficulties or read a grammar book all the way through. It may be boring, but it's extremely helpful.
Practice writing! The more you write, the better writer you'll become. Every skill requires regular repetition to achieve mastery, whether playing a musical instrument, flying a jet or writing good English prose.