This will depend on the league you are coaching in and at what level you want to coach. Most leagues require no certification to coach at the Recreational level while you'll need a certificate/license to coach at the competitive/travel level. A number of recreational leagues are requiring coaches to attend short 6 hour classes before coaching (USSF Youth I/II). If you plan to coach a travel team, the entry level license is a USSF 'E', followed by a 'D' license. These are administered by your state soccer association with a curriculum created by the USSF. If you chose to coach at even higher levels, there are residential classes for the 'C', 'B', and 'A' national licenses, taught by the USSF. Finally, there is the USYSA/USSF National Youth License - a week long course designed for Directors of Coaching and others responsible for the development of youth soccer training programs - but it is an excellent course for committed youth coaches as well. However it is expensive (over $500) and will either take a solid week or two weekends (Friday evening through Sunday). See the USSF Coaching Education Page for more information. The National Soccer Coaches Association of America also has a coaching education program with many courses that are 'equivalent' to the USSF licenses, but check with your local and state association to see if they are treated that way.