Look at the Becoming a Train Driver thread on here. Search through the forum for other people who've asked the same question in the past.
A part from that, keep an eye on local train operators careers pages on their website for when vacancies come up. Trainee driver roles are quite rare and in high demand, but you could conciser another role on the railways too.
To be fair, there's some really good advice on this page. When I first applied for a trainee role i spend pretty much a whole afternoon trawling through the pages on this forum for advice, alot (but not all) was good.