If you are serious about writing a novel, novella, a book of short stories or poems; create a goal for yourself for each day. They say in order to create a habit, one must do the same thing for 21 days (maybe longer depending on the individual). Write everyday and hit the goal you set for yourself. Eventually, sitting down to write will become second nature.
However, it’s tough sitting down to a blank screen because, when this happens, many times the mind goes blank as well. Help your mind out by creating some stimulation for it. Again, we’re all different, so the type of stimulation will be different for everyone. Maybe music will get your mind going, or even the television in the background. Whatever works for you. Or, just start writing what comes to mind. This gets those wheels turning as well. Before you know it your fingers will be flying across the keyboard.