Read stuff. Then read stuff. Try traditional books and "real" newspapers. Then read some more. After all that, get another real newspaper and read that.
You'll be amazed at how quickly your grammar and spelling will improve.
Honest, try this for just 6 weeks.
Here is a site that may help.
http://www.spelling-words-well.com/spelling-help.html
Also if you are having a problem, you might want to be checked to see if you are dyslexic. With a proper diagnosis you can be taught how to deal with it.
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