For individuals, the Tax ID number is usually a social security number.
Folks that don't have (or aren't eligible for) a social security number may have a Taxpayer Identification Number (TIN) issued by the IRS.
Businesses often have an Employer Identification Number (even if they don't have employees).
If you believe you have an EIN and have forgotten it, call the IRS at (800) 829-4933 (7:00 a.m. - 10:00 p.m. local time, Monday through Friday). They'll ask some questions to verify your identity, then provide you with the number.
If we're talking about a sales tax number, you want to contact your state department of revenue.

