If you're building a first eBook and are game to tackle the work yourself, browse over to Paul Salvette's web site. Paul has plenty of advice, guidance and additional links to assist you with the process. I'll toss in: get ready to get your hands dirty. With patience, effort, and Paul's coaching though, you can prime your masterpiece for a professional-grade release via Kindle, Smashwords and the other e-venues.
Maybe you aren't too worried over the finer points and are willing to accept 'good enough' quality. That's your choice and certainly there are shortcut methods you can resort to. Another alternative is you can just pay to have your eBook prepared. Plenty of wranglers are available for a modest fee.