I wouldn't prune during the growing season, except maybe to snip a partly broken branch or something like that. In late winter, but well before the buds start to swell, do your pruning. Select 3-4 branches, or whatever is fitting for the type of trellis you have and remove all other branches. Then remove all but a handful of buds on the branches you've selected.
It's scary to prune grapevines, but if they have good healthy root systems, they will respond with awesome production. If you don't prune, or don't prune enough, they will just give you tiny grapes and a lot of brush. If you ever have a chance, look at a commercial vinyard in March to see how they prune. You'd think they are killing the vines - but it doesn't happen. Instead the vines bear ever more fruit.
