The teacher should be entering you. However, if he doesn't have a teacher, then enter via the exam board. Are you doing ABRSM? If so, use the link above. When you enter, choose "at a centre" and a drop down list of centres will come up; choose your nearest.
Worth knowing that you have a bit of time yet; exams are held in local centres 3 times a year and your next chance to take it will be in the Autumn, late Nov/ early Dec time. The closing date will not be until mid September. So the facility to enter is not yet online; you will still only see for period B which has come and gone. You will need to wait for the online form for period C comes up. You'll need to email for an entrant number first, do that now and then check back in September to make your entry.