I'm not a professional writer, but I have taken many many hours of writing classes, so let's see if I can help. First of all - read more. Read the type of writing that you wish to do yourself. Grammar is important, however, it is not as important as creativity. I would have to say that the only real way to improve your writing is to WRITE. Write every day, as much as you can. Post it on here and ask for criticism. Find other writing forums and do the same. Here is a Google link to some screenwriting forums, however, I'm sure you could find some more suited to your preferred style of writing.
Some screenwriters' forums:
The other important thing is to learn to quiet your inner critic. You know, the little angry person in the back of your head who is constantly spewing insults your way like, "Oh this is ridiculous, no one wants to read about this" or "Are you kidding me? This has been done so many times before". Once you start learning to let words just flow onto the page without editing as you write - things will go much more smoothly. You can always edit later! No one has to read everything you write. JUST WRITE :)