You need to apply for the ITIN together with your return and send it to the ITIN operations center.
See the instructions of form W7.
Form W7:
www.irs.gov/pub/irs-pdf/fw7.pdf
Instructions for Form W7:
www.irs.gov/pub/irs-pdf/iw7.pdf
Once the ITIN is assigned, they will forward your return for processing and send you a letter with the ITIN.
Getting the ITIN takes about 6 weeks. Processing the return can take another month or two.
http://www.irs.gov/Individuals/General-ITIN-Information
How and when can I expect to receive my ITIN?
If you qualify for an ITIN and your application is complete,
you will receive a letter from the IRS assigning your tax identification
number usually within six weeks. If you have not received your ITIN or
other correspondence six weeks after applying, call the IRS toll-free
number at 1-800-829-1040 to request the status of your application if
you are in the United States. If you are outside the United States, call
267-941-1000 (not a toll-free number).
