First, when and why you need to give your Social Security number (SSN) to someone:
- When starting a new job, you'll need to give your SSN to your employer so that they can report your income to the IRS and wages to the Social Security Administration.
- When opening an account with any U. S. financial institution.
- When applying for a federal loan, such as a federal student loan.
- When applying for certain types of public assistance, such as unemployment benefits.
- When enrolling in Medicare (you apply for Medicare through the Social Security Administration).
- When applying for a passport.
- On your tax return, so that the IRS can match the income you report to that reported by your employer(s) and financial institutions.
- To get a driver's license.
As for when NOT to give out your SSN, as well as more complete explanations of the above, check out this article.