See the information about EINs at the following link on the IRS web site.
https://www.irs.gov/businesses/small-businesses-self-employed/employer-id-numbers-eins
You don't need an EIN unless you have employees, or for certain types of self-employed retirement plans. But you might want to get one, even though it's not required, if you have clients to whom you have to give your tax ID number. By giving them your EIN you can avoid giving out your Social Security number.