There are a bunch of online resources you can use to improve your spelling.
Just google "improve spelling"!
Here are some links I found which may be helpful.
Spelling it Right - Learn to spell confidently
Learn English - English Spelling Guide - How to improve your spelling
Improve Your Spelling ę LearningNerd
Yahoo! Answers - What can I do to improve my spelling?
You may also want to use a book:
Spelling Success Express - General spelling helpers
Hope this helps!!