Do I have to make a reservation to take the monthly test?  

If you're taking the normal monthly math test during the normal math test week (the week when the work is turned in), then no reservations are needed.  Just come to your normal class time; you will take the monthly test during class.

If you are going to retake a test, then some of the times that are available for retaking a test do require a reservation.  Go to the webpage that explains when to retake tests.