Leading a national team is a large privilege and an honour given only to only the best experts valued in the country or leagues, capable of selecting the best players born in a given country. In FIFA 14, you will receive such an offer after achieving good
Leading a national team doesn't involve many duties - apart, of course, from playing the interstate matches and trying to meet the requirements set by international federations. You need to select a dozen or so players which you will be using during the games. You have a free choice, while remembering about the fixed number of players for given formations. Remember about diversity, as you cannot appoint more players when one of your basic ones gets injured.