Most graduate and professional programs require applicants to submit test scores from at least one graduate admission test. This score(s) helps the programs to which you have applied predict your academic success in their program if you were to be admitted; this is similar in nature to how the SAT or ACT scores were used in assessing your strength of candidacy when you applied to your undergraduate institution.
What test or tests you need to take vary in the types of programs to which you are applying and can vary from school to school. Each school has their requirements which you will want to familiarize yourself with thoroughly before beginning the application process.
Here are some graduate and professional assessments to look for as your read over application requirements:
You will need to register to take entrance exams. Testing centers are located throughout the United States. It works well to identify a testing center located close to where you are enrolled in school or where you are currently living/working. Make contact with the center of your choice and follow their registration processes for taking the exam you need.
Take the exam(s) you need with plenty of time to have your test scores sent to the schools to which you are applying well before their deadline. Your application materials will not move forward in the application process until those test scores are received.