Hi Darren,
If you are planning on going into military flying, the UK services have a yearly selection round for TPs - the minima are 700hrs P1, 2 tours as Captain and an above average rating (a grading which is a classic piece of British Understatement). I think most other countries have a similar approach - there was a chap on my Syndicate on ETPS from the USN who was given a choice of TPS or Top-Gun. The TPS course itself is a vicious piece of ritual torture lasting about 1 year, but leaving you knowing far more about aeroplanes than you thought there was to know.
In the civil sector, there's no straightforward route (other than being an ex military TP), but most people who find themselves in test flying are experienced high ability pilots, who have also got a very good technical background (the ideal would probably be a qualified avionics technician who's also got a degree in aeronautical engineering).
But, the bottom line is that nobody trains and goes straight into flight testing - it's something that comes after a lot of other experience. However, those who make it often go through their early careers with a firm eye on that possibility.
I hope that helps, but by all means ask more specific questions if I (or other silent presences lurking around Pprune) can help.
G