Often a master's program will require that you complete a minimum number of course requirements during your undergraduate career within the relevant field. Some programs require a certain number of credits in specific courses of study. For example, if you are applying for the Master of Social Work program at the University of Michigan, you may be required to have completed a human biology course. Check with the school's website or contact an admissions representative to determine if you meet all requirements.
Master's programs often require that prospective students take a certain standardized test and submit their score directly to the school. The standardized test, while not perfect, serves as a way to compare relevant skills from diverse students from varied backgrounds. For example, if you are applying for a master's program within the humanities such as history or English, the graduate school may require scores from the Graduate Record Examination (GRE). The GRE tests your reasoning abilities, vocabulary and basic math skills. Some schools may require a minimum score on the applicable standardized test.
Most schools these days have online applications that can be submitted electronically, though you may want to check with the school to see if they prefer paper applications. Fill out the forms to the best of your abilities. Be sure to be completely truthful when answering the questions on the application. If any of the answers are found to be false, that may disqualify you for admission.
Often, applications will need to be accompanied by a fee, a resume of your academic and professional experiences, and an essay. For the essay, the topics will differ based on the program to which you are applying. Be sure to address the essay stimulus questions fully and always proofread materials before you submit them. You may wish to ask a trusted friend or professor to look through your application before you submit it.