Employer Identification Number (EIN)
Unless you are starting your practice as a solo, and you plan on having no staff support (e.g., perhaps working from a home office), you will almost surely have to apply for an EIN. If you are not a sole proprietor, you will most likely need an EIN regardless of whether you have staff. Visit the IRS site to find out if you need an EIN, and if so, whether you are eligible to apply through the IRS' online application.
Federal Licenses and Permits
Law firms should not require a federal license or permit unless they, for some reason, are involved in a federally regulated field such as securities, broadcasting, or transportation.
For more compliance information, visit www.business.gov.
Federal registration of intellectual property, including patents, trademarks, trade names, and copyrights, provide business owners with exclusive use of intellectual property in the U.S. as well as in a large number of foreign countries. See FindLaw's Intellectual Property section of the SBA site for details.
State Guides to Obtaining Licenses & Permits
The following links will direct you to information on how to obtain licenses and permits for your new firm in each of the 50 states and Washington, DC.