If you want to apply for accommodations for your student or yourself, you must first become acquainted with the criteria being used to evaluate your application by accessing the appropriate website.
For the College Board, click http://student.collegeboard.org/services-for-students-with-disabilities/eligible-for-testing-accommodations
For the ACT, click http://www.actstudent.org/regist/disab/opt1.html
Students in both public and private high schools may apply through their schools if they have valid, current evaluations documenting the need for accommodations according to the stated criteria.
For those who have never been evaluated, parents may seek to obtain a private evaluation to establish the appropriateness of the request for accommodations. Again, choose an evaluator who is very familiar with not only the criteria, but also how to deal with/write an appeal to the respective Boards if they deny accommodations when you and the evaluator feel they are warranted.