Canon Law, the Church's official legislative text, gives each local conference of bishops the authority to decide the specific requirements for Catholics in their territories, so the United States Conference of Catholic Bishops (USCCB) has determined the norms that we must follow. The USCCB outlines the following:
Ash Wednesday and Good Friday are obligatory days of fasting and abstinence [from meat] for Catholics. In addition, Fridays during Lent are obligatory days of abstinence [from meat].
For members of the Latin Catholic Church, the norms on fasting are obligatory from age 18 until age 59. When fasting, a person is permitted to eat one full meal, as well as two smaller meals that together are not equal to a full meal. The norms concerning abstinence from meat are binding upon members of the Latin Catholic Church from age 14 onwards.
Although the US Bishops document warns that "no Catholic Christian will lightly hold himself excused from this penitential practice," there are reasonable exceptions to these requirements. Those with a medical reason not to fast or abstain from meat, for example, may be excused from these obligations. If you are in doubt, talk to your pastor - Canon Law 1245 gives pastors the authority to grant a dispensation from the requirements of fasting or abstinence.
If you're interested in diving more deeply into the documents that officially outline these requirements, here they are: