As
@J3053B has said, there isn't too much you can do to prepare as everything will be taught to you from the very basics, however...
For me, it was around four weeks in a classroom at the Network Rail Training Centre in Basingstoke. The first few days are just a corporate welcome. You'll receive some freebies and it isn't too intense.
After that, you'll move onto the "proper" stuff, starting with PTS (Personal Track Safety). There's an exam on that which is pretty straightforward.
Then comes the really heavy content, and you'll be taught about signalling, dispatching, train accidents and all the rules surrounding them.
You'll have a few "out days" interspersed throughout the course, which allow you to stretch your legs away from the classroom. On my course, we were taken to Alton for ramp laying practice, up to the operations centre in Basingstoke for a brief tour and there was also a "train working day" which allowed us to put into practice what we'd learnt thus far in the classroom.
The pace picks up as you near the end, and progress assessments will be thrown at you left, right and centre, but this is all good practice for the final exam you'll have to sit.
After you're done with the classroom-based learning, you'll spend two to three weeks learning the traction (partly as static in the depot, and then train working and being assessed on that). After this, you'll return to Basingstoke for your final rules exam, and then you're off to your depot for route learning, etc.
Your commercial classmates will return to Basingstoke in a couple of weeks for their commercial course, but as a metro guard you won't have to.
There's no denying it's tough and intense - it's certainly a heck of a lot to take in, in such a short amount of time - but I found it interesting and really good fun.
The exams seem daunting at first, but they're mostly multiple choice and given you've paid attention and done your revision, you'll fly through them.
Good luck.