Hello and welcome
This is awesome imo http://www.youtube.com/watch?v=8OaicleoK4M
There's a good recent thread about ukemi here with lots of advice and more videos etc.
If you're hitting the mat hard remember to get as low as you can before you go and also that the feeling is more horizontal than vertical so give yourself a good push off rather than collapsing. Also try to relax, jiggle around a bit first to get that loose feeling, and try to imagine yourself as a big springy ball.