Apply online at www.mass.gov/civiliservice by March 1st, 2016 (there is an additional charge for exams taken after this date, and no applicants accepted after the 21st).
The date of the exam will be April 16th, 2016. This exam is not free. It will cost $200.
Or call 617-878-9895 if you are in Boston. If you are outside of Boston you should call 800-392-6178
Some tips: Visit that website. Read everything carefully. Don't be afraid to ask them questions if something does not make sense.
And I wish you the best of luck!