The Department of Commerce is the principal government agency responsible for managing the nation’s Import and Export function.  Detailed information about what is required to conduct Import and Export business is found at their website.

When you buy a product overseas and import it to the U.S. the vendor that you are buying from will complete the paperwork so that the goods can clear Customs from the country it is coming from and arrive at Customs in the U.S.  From there you will need what is known as Forwarders or Brokers to complete the necessary paperwork and clear the shipment through U.S. Customs.  After it is cleared for shipment it then has to be delivered to your destination.  For this you will need to arrange for transportation.  Most Forwarders/Brokers in the U.S. also perform this function. 

If you import regularly you will have to file some paperwork with Customs.  This will ensure that future shipments clear Customs smoothly.  Your Forwarder/Broker can help you with this.