Like many things in this world, the more you study and practice writing, the better you will become at it. I have written scores of documents, papers, poems, stories, and books, and I find that I still have much to learn. Learning how to write is a never-ending process. One of the best ways to become a better writer is to take some quality writing classes.
Higher Education. Certainly the most favorable sources for improving your writing are colleges and universities. There are many reputable institutions of higher learning that offer excellent writing courses and degrees. Obviously the higher the quality of the college or university, the better your writing course will be. Investigate the colleges and universities nearest you to find out the availability and cost of their writing courses.
Online Courses. There are also countless online classes and workshops for writers. These classes offer everything from advanced writing training to beginning writing instruction. Be careful, though--some of these are low-grade courses, designed mainly to take your money. Make sure you research the different writing courses and determine the quality of their training and their reputation in the writing industry.
Sometimes the best writing class can be a book. Here is one of the best books you can ever read to learn how to write effectively:
The Elements of Style. A fundamental resource for writers, editors, and students. This is the best single source for anyone who desires a pithy, lively guide to the essentials of effective writing. It addresses how to apply the basic principles of grammar and punctuation, properly construct a sentence and a paragraph, use words and expressions in their proper form and context, and avoid the mistakes and mannerisms that make for turgid, pretentious, or simply unintelligible writing.
Here are some great online directories of writer's classes and workshops: