The minimum is 15-20 hours a week for a student visa (in the US, it differs from consulate to consulate).
If you are a US citizen, to apply for a visa, you need to get an official certificate of enrollment from the school to apply for the visa--along with a police clearance, proof of health insurance, proof of adequate funds (bank statment) and a medical clearance from a doctor stating that you don't have any communicable diseases. You also need to have purchased round trip tickets.