An EIN helps identify your business for tax purposes. You need it to file tax returns for your business, run payroll, open a business bank account, and hire employees.
You can apply for an EIN online with the IRS.
When applying for an EIN, make sure to apply for just one EIN! Multiple EINs can cause confusion for both yourself and tax preparers when they go to help you file each year. An EIN should be applied for when there is a separate business operating from your current one.