And that's all that matters. Don't listen to the naysayers (there will always be naysayers). Just do this:
1. Get it all out without worrying about correct grammar, punctuation, formatting. Just get it down on paper. The hardest part isn't starting to write a book, but completing it.
2. Once your first draft is done, THEN start editing. This is the most difficult part, the most tedious...and can be the most fun. Hone your story till every word is perfect, the story well paced, and (most important) you are happy with it.
3. Read a lot in your genre to get a feel for how well your story fits.
4. Once you feel your story is dead perfect, don your flame-retardant armor and get it out there. Join a reputable critique group, either in person or online, for feedback.
5. Ignore the feedback. Just kidding! But don't take every comment to heart--not all suggestions are gold. Listen to what resonates and leave the rest.
6. Revise, revise, revise.
7. If you want an agent, start querying. If you want to self publish, investigate your options for that.
8. Be prepared to revise, revise, revise.
9. BE PROUD of yourself, because whether or not your book ends up a best seller, you did something that most people never accomplish: you wrote an entire book, start to finish.