First--Find books or magazines that are good examples of how you would like to write and immerse yourself in them. Read as much as you can find. Use a local library if you can.
Second--Write a lot. With practice, you will be able to get the techniques you pick up by reading ingrained in your head and eventually develop a "voice" all your own.
Third--Get some good reference books to keep handy for spelling and grammatical questions (dictionaries and grammar check in word processing programs are useful but don't help you learn as much as they just try to correct errors). One possible suggestion:
The Gregg Reference Manual by William Sabin. A book I just spotted on Amazon.com that looks promising is
Reviewing Basic Grammar: A Guide to Writing Sentences and Paragraphs Amazon.com: Reviewing Basic Grammar: A Guide to Writing Sentences and Paragraphs (7th Edition): Books: Mary Laine Yarber,Robert Yarber
Fourth--Some colleges offer non-credit classes to the community for lower tuition costs. See if you can find some on improving writing skills. A brief Google search on "improving writing skills" turns up a lot of possible resources also.
Good Luck!