There's a wonderful book called The Elements of Style by Strunk and White that is considered a classic discussion of how to write simply and clearly. You can find it easily and for free on the internet. Here's a link: http://www.cs.vu.nl/~jms/doc/elos.pdf
An excellent site for help with grammar is this one (notice the "Index" button in the middle of the page). This site will give you lots of interactive exercises:
A good word processor should help you catch spelling errors.
Finally, I would recommend reading almost anything by George Orwell. He is known for his exceptionally clear style, and he discusses ways to achieve such a style here:
Good luck to you!