Starting from scratch with a new teacher is probably best anyway. Check with any local IDTA dance schools. They will probably have a senior teacher, preferably a Fellow of the society, who is responsible for training for professional exams. They will also be able to arrange for you to join in other teachers sessions to help and gain experience.
You don't need a partner to train with. Most of the training will be on theory and knowledge of the technique manual (Guy Howard's book for the IDTA).
Unless things have changed recently you will not have to dance with a partner as lady, but you will be expected to demonstrate solo as both man and lady. You can expect most of the questioning during the exam to be on the lady's part assuming you give a faultless demonstration as man.
Give some thought to the type of pupil you plan to teach. There is a general assumption that those entering teaching later in life will tend towards adult pupils. If you plan to tear around the country taking a bus-load of children to comps every week you may need extra training.