writing takes practice. You need to write every day to build your writing fluency. Daily practice might include keeping a journal or blog. If you have never picked up the habit for writing, I highly suggest using 750 words.com to do intense free writing (with little regard for proper grammar and spelling) for at least five days a week, for as long as you can. The biggest hurdle to writing is getting over the hump of not doing it.
1. To be a good writer, you need to all also be a good reader. Reading well-written books, blogs and articles, and paying attention to how writing is done, can have a direct impact on your own writing skills. Again, most of the reading assignments in school are complete turnoff for students. I highly suggest seeking out topics and authors that you enjoy reading, and not the ones your teacher assigns.