First off, you should read any book by Dan Kennedy. He is an expert on copywriting, specifically Direct Response Writing (where the reader can immediately respond to the offer, such as direct mail advertising). The Ultimate Sales Letter is a great book to start off with.
Visit your local library, there you should find one or two books on copywriting. The Internet is another great resource. Here are a couple of good sites that have great tutorials: www.sherus.com/business/copywriting/ and www.adcopywriting.com/Tutorials_List.htm. Don't try to learn everything at once, but instead follow the step-by-step tutorials available. This way you can learn on your own schedule and according to your own pace.
It is pivotal to write every day. This will consistently help you become a better writer. You must also be constantly learning about your field. There is so much information out there, so don't be worried about learning everything over night. It will take some time and persistence to develop greater expertise.
Hopefully you found this article helpful. You can use books and the Internet to help you build up your expertise. There is so much information out there that you can find material that will best fit your learning style. Copywriting is very rewarding and flexible, so good luck! Improved writing will help you in all arenas, such as websites. Poor web content will keep you from selling your product or service very well, despite the quality of the product. So you must take the time and effort to learn how to create good copy.