Here goes…some pointers for you!
1 – Learn on your own bike. Sounds simple, but it's great advice really. Get confident doing the simple things, like brakes and gears. Then, move on to learning how to true wheels and suchlike. There are plenty of great guides on the net, but might also be worth geting a good bike maintenance book – Clicky
2 – Practice, practice, practice. Service the bikes of your friends and family. Do it for free, as the learning and skills will really help.
3 – Befriend your local bike shop(s) – Pick the one you shop with and let them know you're keen on developing your skills/career in the trade. Many shops will let you look over the shoulder of their mechanics and pick stuff up. Some will then develop to offer you a bit of part time work, building bikes for stock, for example.
4 – Did I mention practice? The only way to get good is to keep on doing it!
Good luck! The bike industry is some of the hardest work I ever did, but also some of the most fun.