The Kindle Previewer available from
http://www.amazon.com/kindleformat/kindlepreviewer is the best tool to convert to the MOBI format.
The Kindle Previewer accepts ".html" files as input. You stand your best chances with that format unless you already have an EPUB version of your eBook.
You’ll find details on "How to make books available for the Kindle platform" in "http://kindlegen.s3.amazonaws.com/AmazonKindlePublishingGuidelines.pdf".
HTH.
Cheers,
Robert