You won't necessarily file jointly. You first need to know if she is a resident alien or non-resident alien for tax purposes. Use the substantial presence test here https://www.irs.gov/individuals/international-taxpayers/determining-alien-tax-status
If she is a resident alien: then she must file a US tax return. All her world-wide income is subject to US taxation with a credit for any foreign taxes paid. In that case you will probably find joint filing has lower overall taxation that separate filing. You get the ITIN by printing your tax return, attaching a form W-7 with the required proofs, and mailing it to the IRS. (She is not your dependent on a joint tax return, but your return would include her personal exemption.)
If she is a non-resident alien: you have two choices.
1. You file married filing separately and report only your income and deductions. She can't be your tax dependent because she is not a US resident for tax purposes. If she had US income she would file a 1040NR non-resident tax return. Turbotax does not include this kind of tax return. Her India income is not taxed in the US. For your married filing separate return, she does not need an ITIN. But you can't e-file, you have to print your return, write "NRA" in the space for her tax number, and mail your return to the IRS.
2. Or, you can make an election to treat her as a US resident for tax purposes. You can then file jointly, but you have to report her India income and pay US tax on it, although there is a credit you can take for taxes she pays in India. You would apply for her ITIN with the form W-7 as described above. https://www.irs.gov/individuals/international-taxpayers/u-s-citizens-and-resident-aliens-abroad-nonr...
Because joint filing subjects her India income to US taxation, you may not want to do that, if you have the choice. You would have to test different scenarios and see which is better.
