It’s not about you. Your purpose is to serve mankind in general, but you have an idea of a set of people you can help. Focus on them.
Get support. Take a class. Start a writing group. This is a team effort.
Immerse yourself in the presence and books of those who have done what you aim to do. Audio books reach especially deep. This step is often overlooked, but you wouldn’t aspire to be a baseball pro by hanging out with basketweavers.
Share your writing, bit by bit. Post on a blog. Read what you’ve written to another writer. This helps overcome perfectionism, and thus writers block.
Get it in your schedule. Make a goal of words or chapters per week. Use the strategies that help you exercise, eat better or save money. Make a chart. Put appointments in your calendar.
Learn what works for you. Make a writing grotto. That is, figure out if you write best in a coffee shop or at the kitchen table. Try the library or a late or early writing appointment. Keep trying variations until you are making consistent progress.