I´ve heard Amazon is pretty good, giving you 70% revenue and other benefits, but honestly i have no idea how to tackle this.
Things you may not have thought about:
1. Get a good quality cover image, pay for one if you have to,
2. Write an interesting and compelling blurb,
3. Have other people read your work before you publish, if you can afford to hire an editor.
1 and 2 are the first introduction to your book so need to catch the eye of the potential reader. 3 is vital, especially in longer works, as you will miss things.
What's the book about?
Oh and good luck!
There's a few things I forgot to mention.
You will have to do your own marketing. None of the self-publishing services will help you with that.
A Good Reads[0] account can help with that, but I haven't had much success with it myself. If you figure out a good marketing strategy please let me know :)
If you're outside of the US and are charging for the book then both Amazon and Smashwords require you to fill out US tax forms as they're both US based. Amazon have a wizard which will help walk you through this. It seemed really daunting when I did it (I had to phone a US tax office for an EIN number) but was actually quite straight forward.
If you've got any specific questions, let me know.
I primarily use lightning source for print books (both for getting copies for myself and delivering to conferences and for distribution to retail and amazon). I used Lulu for my first book and was disappointed by their responsiveness in case of problems (note this was in 2007/8, so things probably changed since). I've published two books through lightning source and those got into Amazon very quickly, and have been selling quite nicely.
For e-books, I use a mix of things. The Kindle store is #1 in terms of revenue for me, and I guess not having a book there is akin to creating an IOS app and not publishing it on the AppStore. Having a book there is a must for discoverability. I still use Lulu.com to get things on the apple iBooks store and other lesser ebook stores, and to sell the PDFs online. The amount of money I earn from those places isn't enough to justify bothering with each individual submission. For the Kindle store, the submission process is pretty straightforward and there is no need to pay Lulu commission.
My two latest books are in progress on leanpub. I think this is a pretty good early-access channel. They now have github support and will generate PDF, MOBI and ePub, so this fits nicely with my workflow and toolchain. Leanpub didn't do anything special in terms of discoverability or promotion for my books, but they were a good channel to republish stuff frequently and let people buy early versions. I plan to re-do the PDF completely for print publishing, but I'll use the MOBI and ePub they generated to sell the books directly on the Apple iBooks and Amazon Kindle stores. I think that's going to be better than just keeping it on Leanpub, again because of discoverability. So if the book is already done, I don't think you'll get much from them.
Smashwords.com is another option that looks popular with fiction writers, but I never really did anything there so I can't provide much more than a pointer.
70% of my book sales come from Amazon. Smashwords gives me more $ per sale.
If you had to go with one service first, I'd recommend Amazon Kindle Direct. Once you get comfortable with how the process works, branch out to Smashwords if you like. It took me less than a day to get set up and they have pretty good documentation.