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.
How do I file with a W-7?
Then, complete your return as you normally would. Leave the applicant's Social Security Number field blank, and disregard any warnings about the empty field or missing health care info for the applicant.
When your return is finished, go to the File section and follow the steps to print a paper copy of your return, as you won't be able to e-file with a missing SSN.
Bring your printed, signed return (along with any attachments per the filing instructions, such as W-2s or 1099s), the completed W-7, proof of identity, and foreign status documents to your acceptance agent or an IRS Taxpayer Assistance Center.
Alternatively, you can mail everything to:
Internal Revenue Service
Austin Service Center
P.O. Box 149342
Austin, TX [social security number removed]
See also General ITIN Information at IRS.gov.