CBP recommends that importers review the topics on the CBP import page. In particular, CBP suggests viewing the information contained in the section titled Basic Importing & Exporting.
There are many topic-specific links to explore, but you may wish to start with Tips for New Importers and Exporters. This will lead you to information on CBP import requirements, arrival of goods, formal entry vs. informal entry,classification, protest, mail shipments, restricted merchandise and more. For other agency requirements you may need to meet, and if you become a frequent importer with higher valued shipments, we recommend you read Importing into the United States, A Guide for Commercial Importers (in PDF).