I'm not sure this is still required in the states but here's what I did years ago.
You need to get a Restricted Radio Telephone OPERATORs License.
Here are the instructions from the FCC website:
http://wireless.fcc.gov/commoperators/rp.html
You must first register and establish an account:
https://svartifoss2.fcc.gov/coresWeb/regEntityType.do
or go directly for FCC form 605 to apply on-line (and establish an account). You use form 159 to pay the FCC.
Forms | FCC.gov
Read the instructions carefully! You shouldn't have any issues if you do.