You can find out more information in the 'Education' section of the BHS website - found here.
There is a link to the syllabus which tells you what you'll need to be able to know/do for the exam.
When you say enrol, do you mean enter the exam? You do this by filling in the application form on the page I've linked ^.
If you mean enrol on a course, you do this through the centre at which you wish to train. There's a list on the BHS site of where to train.
Hope this helps!
eta - You don't HAVE to train at a centre, you can enter the exam without formal training, though most people will say that this is inadvisable!