Teaching yourself to write can be extremely difficult, however, the first step to good writing is good reading. Learn from what you read and try to read daily. Through the experience of reading both good and not-so-good pieces (Gone with the Wind or the National Enquirer?) you will understand more than just the content. You will learn by example the rules of good grammar, usage, and punctuation.
Right now, I am reading George R.R. Martin's Clash of Kings, the second book in the Game of Thrones series. Although this book is almost a thousand pages long, I am thoroughly enjoying it, both for the story and the exceptional example of writing it represents. I firmly believe that by getting into the habit of reading every day, as well as encouraging our children to do the same, our writing skills will be greatly improved.