I think you should start by reading the FAQs, browsing this forum, and downloading the new Kindle publishing guide to your device.
I prefer to work in html because it gives me near-infinite control over how
my book will look. There's a learning curve, but the same is true of any
method for submitting books to the KDP. I'm told Open Office does a better
job than Word of saving text as html. Better yet, paste a Word doc into the
utility at word2cleanhtml.com (requires some tweaking).
I just noticed that CreateSpace now offers a Kindle ready version of the book for download along with a jpeg version of the cover under "Publish on Kindle". I used the cover download, and it seems to work fine. I'd be interested to see if the file download converts successfully.
I'd recommend publishing on Kindle BEFORE you publish on Createspace. For one thing, sales are likely to be higher with ebooks. Also, you will inevitably find things you want to change and it's easier to do with an ebook. You also might find you're not selling at all, in which case you'd want to try and address that before investing in a proof copy (a step you should never skip) of your Createspace book.