I've been writing since I was very young. Honestly, its true what they say. Just as we grow as individuals as we age and experience life, so too does our craft as writers mature as we work at it. Most writers have to work at or massage the text to make it the way they want it to be.
I once came across some prose I wrote back before I was even in High School. It was utter crap. Later found some writing from nearly 14 years ago, it was crap too. What does that mean? It means we learn as we do as writers. There is no substitute for writing.
So you get an idea what do you do? First carry a note book and pen with you at all times. I have a little pocket size compositions book I carry with me and try to jot ideas down in for writing. That helps a ton. Write till you've got the ideas out of your head. Then later pick them up, revise, expand or ignore if you decide they aren't worth continuing.
Then when you find time, write, write about whatever comes in your head. I once saw a guy who wrote fifteen hundred words just on the grooming routine of this one character. It was crazy! But at least he was writing.
I've been writing, for quite some time now, and while I've not had anything yet I want to publish, I've found that I have grown as I've written. I've mentored other young writers, and I always tell them to write as often as they can. The more time you spend writing, the better it becomes.
As for how to improve your writing? Get a grammar handbook. Review the rules of the English language. Then read! Read a lot of different genres and styles. There are so many ways to express an idea. Relish it, experiment, and then don't be afraid to try new ideas of constructing your writing when you have the time.
Ultimately there is no substitute for sitting down and doing it. One more thing, sometimes it helps to find a muse, a focus. Maybe its cooking, maybe its science fiction, or history, whatever it is find a subject and start writing. Look online for others who are interested in writing similar things. I for one am part of several groups that write collaborative fiction and we learn a lot from each other too.
I hope this advice helps.