Depends on the colour of the nails. If you are lucky and they are off white/light brown you can see where the blood supply ends and cut say 2mm below that. if they are black and you can't see anything, start by taking 2mm off the end and then gradually work backwards over the next few weeks until they are a reasonable length.If you are unlucky and the nail starts to bleed, simply apply pressure with a cotton wool pad for a few minutes and it will stop.Then obviously adjust how much you take off the other nails.
Having said that, unless they are really long, long nails are unlikely to be the cause of the problem. As stated in another post, boredom can be a major contributing factor in young dogs, especially the more highly strung breeds.