There are many online resources that can help you.
Alison.com offers free courses in writing and other subjects. It is one of the first participants of MOOC (Massive Open Online Courseware). You just sign up and select the courses that you want to take. Everything is self-paced.
There are also top universities who offer MOOC. You have to watch for their enrollment period. It's all free, with an option to get a certificate (for a fee) if you need one.
Don't forget to seek out other opportunities to write. The more you practice, the more proficient you will be.