Foreign students are not allowed to work during the 1st academic year in the U.S.A. There are some exceptions to this law.

With the authorization from your student adviser, it may be possible to work on campus. The work should not displace a U.S worker or interfere with your studies and is limited to 20 hours a week. 

A graduate student can be employed as a research or teaching assistant. A student can also participate in a 'co-operative education program' and 'practical training' that is part of the course of study, after receiving  authorization from the foreign student adviser. But if you participate in the 'co-operative education program' for a whole year, during your study, you wont be eligible for 'practical training' after completion of your degree.

Undergraduate students may be authorised by the foreign student advisor to participate in practical training only after maintaining student status for at least one academic year (generally nine months).

After completion of the degree you can apply to the INS (Immigration and Naturalisation Service) for permission for practical training for 12 months. You must find appropriate employment  within one month after the completion of your degree.