IRS - E-Services - Log-in and view your EFIN, Application Summary Page, add EFIN's, etc. (click the blue "Access e-file Application" button):
https://www.irs.gov/tax-professionals/e-file-provider-services

IRS - Become an Authorized E-File Provider - Get your EFIN and start your own tax prep business:
https://www.irs.gov/e-file-providers/become-an-authorized-e-file-provider

IRS - Tax Information for Tax Professionals:
http://www.irs.gov/Tax-Professionals

IRS - All things PTIN:
http://www.irs.gov/Tax-Professionals/PTIN-Requirements-for-Tax-Return-Preparers

IRS - All things EFIN (click the blue "Access e-file Application" button):
https://www.irs.gov/tax-professionals/e-file-provider-services

IRS - All things EIN (Employer Identification Number):
https://www.irs.gov/businesses/small-businesses-self-employed/employer-id-numbers
https://www.irs.gov/businesses/small-businesses-self-employed/how-to-apply-for-an-ein
https://www.irs.gov/businesses/small-businesses-self-employed/apply-for-an-employer-identification-number-ein-online