You can always work on-campus for pay without special INS permission as long as you have a Social Security number and card.  All other kinds of employment, practical training, or academic training are subject to specific regulations and authorization.  The rules governing employment for international students are complicated and change every few years.  The Director or Assistant Director of the Center for International Education know what is permitted, possible, and legal, or will direct you to appropriate personnel.  You should seek advice before applying for off-campus employment.

Social Security Cards
Students who have been offered on campus employment are eligible to apply for a Social Security Card (also known as a Tax Identification Number). This card will permit you to engage in work in the United States. Students must have a letter from their prospective employer before they can apply for their Social Security Cards. Visit the Center for International Education if you have questions and for the application for the Social Security Card.
Social Security application

On-campus Employment
Students with F-1 status may work on-campus up to 20 hours per week during semesters, and up to 40 hours per week during school vacations without INS permission.  A listing of work opportunities available on-campus is maintained in the Student Financial Services Office located in the Bart Luedeke Center, second floor.  At Westminster, on-campus employment listings are kept in the Career Services Office located in the Scheide Student Center. Please visit the Student Employment section. Before you begin your work, you will need to complete an I-9 and a W-4 form. The I-9 form reports your working status in the US and the W-4 form reports your tax withholding status to the US government.

Curricular Practical Training
Curricular Practical Training is either an internship or a coop that each J-1 and F-1 student is eligible to engage in while they are a student. Some Curricular Practical Training opportunities are paid while most are credit bearing experiences directly related to your field of study.  Each student may engage in CPT for up to 12 months.
Curicular Practical Training Guidelines for J-1 Visa Holders

Optional Practical Training
Optional Practical Training is employment related to your field of study.  Every F-1 student who has been in lawful F-1 status for 9 months (one academic year) is entitled to a total of 12 months of full-time Practical Training in their field of study.  You may pursue Practical Training before or after completion of your degree, with some limitations which can be explained by the Director or Assistant Director of the Center for International Education or other appropriate personnel.  Apply 3 months in advance of your expected graduation.