Your spouse, who is a non-resident alien, could only get a tax number if you are a US person (citizen or green card), and you are required to file a US tax return, and you want to file as "married filing jointly." If you file married filing separately, your spouse does not need an ITIN and the IRS will not issue one. If you do file jointly, you must include all your and your spouse's world-wide income, even though your spouse is not otherwise a US taxpayer. Your spouse would get a tax credit or deduction if they also pay income tax in their home country, but it may or may not lower your overall US tax bill. You would have to test your situation.
If you want to file jointly, you need to fill out a form W-7 application for ITIN. Print your tax return and mail the W-7, the tax return, and any proof of identity required for the W-7, all to the IRS W-7 address. After the ITIN is issued, the IRS will process the tax return.
