If you live outside the USA and do not have an Individual Taxpayer Identification Number (ITIN), Smashwords and Amazon (or whoever you are selling your books through) will withhold 30% tax. However, you are not obligated to obtain an ITIN if you are happy to live with this fact.
If you choose to obtain one, the process is as follows:-
Download a W-7 form from the IRS website along with instructions for filling it out. At the same time, download a W-8BEN form. When you have uploaded your ebook onto Kindle Direct Publishing, you will be able to download a letter from them explaining why you need an ITIN number. Enclose this letter along with your completed W-7 form and other necessary identification documents and mail to:-
Internal Revenue Service
PO Box 149342
Austin, TX 78714-9342
I think this process takes about six weeks. When you receive your ITIN number, you then need to fill in W-8BEN forms with your ITIN number and send them to whoever you are selling through, eg., Smashwords, Amazon.