Help with Wisteria

Please can someone tell me the best way to prune a young wisteria tree. I am growing in a lge pot next to trelis. It has a lovely bushy head, but loads of long roots pretruding from the top. I have been told that I should cut these off and turn the trelis on it's side to allow my tree to flower. Am novice gardener and am not sure what to do.
Thanx xx

 

Try cutting the new green shoots back to downward pointing buds (they are just where the leaves join the shoot), and keep the new shoots short - maybe 4-6 inches only. Keep on doing this and you can't go far wrong.