Read, read, write and read. You need to read extensively until you are sure which genre you want to write in, and then study how good books are constructed, and figure out where the less good books are failing. You also need to write - you will only improve with practice. Words on screen, on paper, on the back of your cheque book - it doesn't matter, get that practice in, and learn as much as you can about grammar and correct sentence/paragraph construction as you can. While you will probably need an editor, the less you provide for them to fix, the less money you will have to pay out up front before you are at a publishable standard. And finally, don't publish too soon. Have your work critiqued, beta read, edited, and then polish, polish, polish before you send your baby out into the big wide, cruel world.