What can I do to improve my writing.?

What I would like to do more than anything in the world is write a book. The problem is that I am nineteen so I feel like it is too late to start having this dream now. In high school I took remedial english cles and worked my way up to honors cles. I feel now that high school didn't prepare me for college writing cles because I had some difficulty in my English 101 and 102 cles. I now feel confused about what I should do now because I feel like my college courses didn't help me either.Is there any way for me to be able to improve my writing skills dramatically in the shortest amount of time as possible.