Depending on which genre you write it varies, but generally, I recommend that you:
1.Start creating your characters (appearances, traits, personality), settings, conflict, etc. before you begin writing rather as it tends to execute better than along the way.
Use accurate grammar that matches either the time period your story is in or if the book is in the first person, by the age or personality of the character. It makes the book seem far more realistic.
Do not make "perfect" characters! It makes it incredibly difficult for you to develop them!
Inspiration. Whether it be by music or another novel etc, try to find something that will inspire you to create.
Grammar assistance applications. I suggest Grammarly.
Write. Even if it's not the best, practice makes perfect.