You may work off campus in OPT and CPT but only in jobs related to your major field of study. These jobs are VERY difficult to find and will be extremely limited due to the US economy. If a company has just laid off skilled employees, they will not be hiring student trainees. Your financial resources to obtain your visa should have been adequate to fully support your needs. You should not have to work for sufficient funds.
Sometimes - I note this is very, very infrequent - international undergraduate students qualify for on campus jobs. Speak with the international office after you arrive.
As an international student, expecting to work 20 hours per week without it affecting your school performance is unrealistic. A 15 credit course load translates into at least 60 hours per week of class and study. Poor performance in class will get you suspended from the university; be realistic in the amount of time you need for study, sleep, travel, and activities of normal living and you will quickly see that your expectation to work 20 hours a week is unrealistic.