It's some awesome, that a lot of people here wrote a book. I would like to write one too, but I think, that I don't have enough experience, or it will suck.
Start writing a blog, now. That will:
1. Give you practice finishing smaller written works before you work your way up to a much larger book project.
2. Let you practice all of the various skills in writing.
3. Improve your writing/marketing based on the feedback you get from people. Publishing on the web is a fantastic feedback loop for learning what resonates with people and what doesn't.
4. Build an audience for your writing. Set up an email list and put a link to it on your blog.
The way you get experience writing is the way you get experience in any other artistic pursuit: practice and feedback.