If you want to be a good writer, study your craft. You wouldn't preform brain surgery without training, don't dishonor your book/story by going in half-assed. Arm yourself with the right tools.
First, get yourself a good grammar book. I personally like Stunk & White. Just the facts here, no window dressing. Grammar is the basics of writing and just like any other skill you can learn it. Refer to the book with any questions that pop up. Make notes in the book. I use post it notes to flag pages where I know I'm weaker. Every now and then I page through it to reinforce those rules in my mind. After all, if you don't know the rules, how ya gonna break them?
Second, Writing Down the Bones by Annie Lamott. This book will show you some ins and outs of the writing life. It ain't all tea parties and book signings. Writers work. Hard.
Third, will give you some inspiration. Stephen King's On Writing. Find out where he came from an the work he did to get there. From disgusting laundry sheets filled with maggots to how he hit it big. Then he tells you about his own tool box of writing. His writing schedule and his self imposed discipline.
The key to writing your book, finishing your book is found in your daily routine. Prolific writers know this in their heart.