It's pretty easy.
1) Download Mobipocket Creator (it's free from Mobipocket.com.)
2) Save your file as .html (just save normally so you have an untouched backup, then hit "Save as..." in your word processing program and save as .html
3) Open up Mobipocket Creator and click "Import From Existing File" and choose .html
4) I usually make a .prc because that's what was recommended to me.
5) Upload right to Amazon. They'll convert it to their format. Check it over and you're done.
Plus, once you have it as a .prc, you can use Calibre or something to convert it right to an .epub